The 2 most common lift kits for the OBS for ~6" are the RCX 6" (~$1000) and the Dick Cepek 6.5" (~$1500). The RCX is actually a 4" lift kit with torqued torsion bars and an add-a-leaf (AAL) for the rear. There is alot of debate over which is better the RCX or the DC lift, but for most its you get what you pay for. $1000 for a 4" lift, AAL, and some tweaking, or $1500 for a full 6.5" lift. DC lifts are practically indestructible and for many people, it's a gateway to a solid front axle swap (SFA). The RCX is rumored to be not as strong as the DC, but I have never heard of either breaking.
Basically its what you want for lift, and how much you want to spend. There are parts you should replace when you do the lift as well, tie-rods etc. that most people also recommend to replace when you do the lift. I am unsure on what exactly the parts are that are recommended to be replaced, but its in the ballpark of ~$500 with Moog parts.
Also, if you are looking at a >35" tire, most people recommend regearing which if done by a shop can be upwards of $1000 for front and back regearing.
Hope this helps!
-Greg.