20k into a rex seems kind of overkill "bro". just my 2 cents, but for 20 grand I would personally invest it in something else if youre serious about that kind of money. like a later generation civic and swap the k series in there. but like GTA said, its your car.
also, like nismo power said, your definitely gonna need something bigger to mess with anyone pushing a decent v8. and chances are, if they do have a v8 and will race you, they're confident that they can beat you, meaning they have invested some money into their car. Make sure you surprise them....