this vehicle slector is pretty awesome, but unfortunately it doesn't work anymore with the lastest GTa IV update (november 10th 2009) which is pretty strict on modding... hopufuly someone will create a new scripthook or dsound.dll to re-enable it.
The options that unlock as you buy cars are the ability to give you cars extra defence against the diferent danger sources like bulles, fire, explosions, ramming, flat tires, etc.
The only things that can be customized are the colors of the car, some of them have decals too, add a pearl/metallic finish to the paint which looks really great, etc
For example, the NRG 900 (the most sport-like and fastest bike) can have two different bodies and several exhaust pipes, you can really choose any you like.
On some of the vehicles changing the body parts (which have really weird names that don't mean a thing) woN't do anything. You have to check them one by one to see if something is available.
It may seem useless but its now, for example if you spawn a Packer (big truck with no box at the back) you have to go in the body options to put a ramp on it. It's pretty fun to spawn one anywhere to place jumps in places there usually aren't any.
That's all I know, hope it helps.