Really, if your gonna buy used, make sure its been bolted on a motor somewhere and it ran good.
They are very streetable, the carbs can be a pain in the ass to adjust out, but the injectors are pretty straight forward. It can be intimadating at times though....
Thats the main thing. the 2x2 weber setups are cool. On the 2x4 manifolds, they give you a lot of tune-ability....me personally, I like the out-to-the-side kits, but be warned, theres alot of tuning involved. IF you have never tuned such a system, I'd take it to a good speed shop. I'd never even attempt to tackle one of these today with out a Fuel/Air sensor tied into the system.
The 8 packs are nice, but expensive.
Also, look for rounding on the screws heads and the nuts,bolts....this usually indicates its been passed around a good bit, alot of folks trying to get it on their motors and running....