Ok, Just to add, concentrate on GETTING A CAR first. It`s ok to get an idea about costs involved in "modifying", but don`t get carried away with the "moddying" idea initially. It will help if you get one which has the potential to easily find body kits, expertise and other stuff in SL (e.g. Starlet GT, Lancer CK2, Civic EG8, Marino, etc.. ). What ever you get, chances are that you`ll need to get some repairs done, so if you intend to keep it for a while, spend on getting things sorted and making the ride clean and reliable first.
As you keep using it you`ll get ideas as to where to start and how to go ahead with adding kits and stuff. Who knows, maybe you`ll change your mind and you`ll want something else before you even begin moddying. But hey, you might be even lucky enough to find one that`s already done-up to your taste as well. Good luck.