I'm always in two minds about this. Some modded cars work out being cheaper, but then again if the work has been done badly it can be costly down the track. I know a mate that bought a car with a fully rebuilt sr20 think it was putting out about 280hp atw. Long story short a couple months down the track the piston rings shit themselves.
I bought what is an almost completely stock car, just exhaust and turbo timer. But I intend on keeping it mostly that way, generally because it is mainly a daily car that occasionally sees some track time. Also because im a uni student so i dont have incredibly deep pockets. If you buy a car that is already modded and putting out huge power you need to have the pockets to finance any repairs and maintenance on a car like that.
On a side note, basic things like coilovers, exhaust etc are always a bonus but I wouldnt pay too much for them because if you need to rebuild the coilovers or something it's gonna cost alot.