Well 1st off you don't have to replace the Bravura, replace them umm... Phoenix for example? That doesn't spawn on the streets at all, only with a cheat. For changing the actual spawn rate I don't remember exactly which file it was, I recently re-formated my PC, if I reinstall GTA SA I'll remember.
For the speed and car handling you use a handling editor (you don't necessarily have to use an editor, but modifying stuff simply with notepad takes experience, and for car handling it takes a bit of automotive knowledge as well). I believe there is one here on TGTAP, but word to the wise though: try not to change the number of gears, I know a Merc with 5 gears is a bit awkward but when I tried to change them I ended up having to double the acceleration just to make the car feel like a car.
PS: This is the right section to post