I like the Cadillacs, but I really don't want a sedan or even a coupe. I want a two seater. The BMW M3 would be an exception, I love that car.
You can definitely spend up to $25,000 on a TT kit and other mods on a 350z, but I would only be putting on rims and body kits at first. I would want to get completely comfortable with the vehicle before messing with the engine at all.