It would be wise to check for modifications already performed on the car. Depending on what was done, it can either:
a) Save you money since the mods were tastefully done and match the level of modifications you were already going to do (things like a rear motor mount, new springs/shocks)
b) Be a warning sign that the car Might have been abused (new intake, downpipe, hacksawed exhaust, slamed to the ground with short short lowering springs)
If you suspect B, its great if you can talk to the actual seller / previous driver since you can actually ask them what they did with the car. They might have actually babied the heck out of it and modified it tastefully. If you are buying it heavily modified from a used car lot, I would stay away since there is a good chance the seller dumped the car due to something being wrong with it. My car is heavily modified and if I saw it on a dealer lot I would run away quick =)