With a 2.5" lift, you should not need anything else aside from the lift itself. Once you up to 4", then the pitman arm, longer brake lines (you can also reroute the stock lines and save some money up front, but the rear you will need - although the better lift systems include the brake line extensions already) and either a TC drop (included in the better kits) or a CV shaft and SYE for the rear (the recommended way to go, but not the cheapest). Other than that, you should be good to go.
Now, you may want to consider some additional things, like new shackles (boomerang for the rear, for example) and such, but it is not mandatory... I also advise to limit the amount of shackle lift, by the way.