You need at least two core units. You need at least three core units for armies of 2000 points.
The models: You get 12 plastic Warriors in the Battle for Skull Pass box set. The Warriors box sold separately contains 16 plastic models that can be built as Warriors or Longbeards. Metal Longbeards models with great weapons can also be purchased in boxes of 5.
Dwarf Warriors are tough. You should always equip them with a shield to make them even tougher. Warrior units can be used for one of two purposes - combat units or support units.
If planning to use the Warriors as a combat unit you should realise that Dwarf Warriors don't deal out a lot of damage against most foes, and rely on outlasting their opponents and winning by static combat resolution (ie ranks, outnumber and banner). To do this you are going to want at least 20 models in your unit (for the maximum rank bonus of +3) and have the full command team of veteran, standard bearer and musician.
The most popular unit sizes for combat blocks of Warriors are 20, 24 and 25. For close combat effectiveness, 20 models is the minimum size to get the full rank bonus. Many players will use a unit of 25 to have an extra rank of models. This enables you to lose some models to enemy shooting before you get into hand to hand and still be fully effective in close combat. Some players will use a unit size of 24 because they plan to add a hero to the unit. This saves a few points but when you place the warrior unit during the deployment phase your opponent will see the gap and guess that you are going to put a hero in that unit.
Most players feel that providing great weapons is too expensive an upgrade to provide to a unit of 20 or more models. Some players have suggested that a small unit of 10 models with great weapons can be effective. This unit would hang back from the front line, with the intention of being used to charge into the flank of an enemy unit once it has got into hand to hand with one of the Dwarf units. If you build your models with great weapons you also have the option of paying the extra points and using them as Rangers.
When used as a flanking unit as described above, the Dwarf Warriors would benefit from shields, musician and veteran but wouldn't need a standard bearer (only 1 standard bearer per combat gives a bonus, regardless of the amount of units in the combat). Having shields provides some extra protection against shooting, and in close combat gives you the choice of either using the hand weapon and shield (S3 attack with 3+ save) or the great weapon (S5 attack with 5+ save).
The other use for Dwarf Warriors is as a support unit that is used for march blocking, charge redirection, guarding war machines, or claiming table quarters. For these roles a unit of 10 Dwarfs with shields and a musician probably offers the best value for points. At 95 points the unit is cheap enough to march across the board into harms way with the intention of disrupting the opponents movement.
Dwarf Longbeards are stronger and better fighters than normal Dwarf Warriors. They are immune to panic also have the Old Grumblers rule, which means they provide assistance for other units taking panic tests. For this reason you may want to use Longbeards as the centre of your army where they can provide the maximum benefit. You can normally field one unit of Longbeards for every unit of Warriors.
For maximum close combat effectiveness you will probably want to use a Longbeards unit of 20 models. Because they cost more points than normal Warriors, you will probably not want to make the unit any larger than 20 models. You should give the models shields and provide the unit the full command team of veteran, musician and standard bearer. It is probably not cost effective to upgrade the Longbeards to have great weapons because then they would become more expensive than Hammerers.
Give the unit a magic standard to increase their effectiveness.
The models: You get 10 Thunderers in the Battle for Skull Pass box set. In the Thunderers box set there are 16 models that you can make as either Thunderers or Quarrellers.
Thunderers has the +1 to hit for the Dwarven Handguns. When comparing to Quarrellers, this means that they will normally hit 50% more of the time at long range (15" - 24") and 33% more of the time at short range (0" - 12"). Thunderers and Quarrellers have the same chance of hitting targets in the 12" - 15" range band. For any range at up to 24" against targets with a 5+ or better armour save, Thunderers are more likely to get through the armour than the Quarrellers are.
Quarrellers are less points than Thunderers, but for shooting at targets less than 24" away Thunderers are still the better unit even taking into account the difference in the points cost. However, Quarrellers can do some things that Thunderers can't, like shoot at targets more than 24" away and they also have the option of great weapons.
The 30" range of Quarrellers means that they should have targets in range from the first turn. The 30" range can also be very useful for tasks like taking out the crew of war machines, or when facing enemy units with longbows that can keep out of 24" range and still shoot at you.
If you decide to give the Quarrellers great weapons, they become a multi-role unit. You could position them a few inches back from the front line. While the enemy advances, shoot them with the crossbows. When the enemy gets close, reform the unit into a ranked unit (ie 5 models wide). Once the enemy is engaged with a Dwarf combat unit, charge into the enemies flank. The Quarrellers are WS4, which is better than many dedicated combat troops, and with S5 great weapons would be dealing out some serious damage. If you build your models with great weapons you also have the option of paying the extra points and using them as Rangers.
If you want one shooting unit, go for Thunderers. If taking a second shooting unit, consider Quarrellers.
Shields are a useful option for both unit types because it will add less than 10% to the cost but will normally reduce your damage by 17%. A standard bearer is not recommended, but a musician can help you rally if you fail a panic test and also can help in close combat. Although a shooting unit, Thunderers and Quarrellers with shields are nearly as effective in close combat as Warriors are and will often hold their own against your opponent's light fast units.
The minimum unit size is 10 models. If you go for units that are much larger, then there can be problems making sure that all the models have range and line of sight to their targets.
If you buy the box set of 16 models I would recommend only assembling 10 or 12 at first. Ten or 12 models is a good size for a unit of Quarrellers or Thunderers. The other models can be used for making engineers for your war machines, replacing the standard bearer in the 10 Thunderers that came in the BfSP box set, or for use in conversions.
The models: Warriors or Quarrellers equipped with great weapons can be used as Rangers. Both the Warrior box set and the Thunderer/Quarreller box set contain great weapons.
Rangers are essentially either Warriors or Quarrellers with great weapons and the Scout special rule. You can take advantage of their Scout ability in two ways.
The safest way is to deploy them in your deployment zone after both armies have deployed their units. This may be to guard a hole in your lines that your opponent looks like they intend to attack, or if you have crossbows it may be that there is a juicy target you want to shoot at.
A bit riskier is to deploy them out of your deployment zone but hidden from the enemy units by terrain. This has the advantage that you may be able to use the unit for march blocking, that is stopping the enemy from making march moves because your unit is within 8". This could slow down your opponents units and force him to either attack with only part of his army or delay his whole attack and allow you more time to shoot him. The downside to this tactic is that the Ranger unit is probably alone and unsupported and may be easy pickings if your opponent sends several units after them. Because of the risk of the unit being killed, giving them a standard is not a good idea.
In either case, because you deploy the unit after normal deployment you have effectively reduced your number of deployments and are more likely to finish deploying first and getting the +1 on the roll off for the choice of whether to go first.