For a nice job 6k will be tight to work with....it's simply going to cost more than his wheels/tires.
You could save some by shooting a bed liner type material on the bottom. How bad did the painter screw up the paint under there? Does it really need to be redone?
Going black and 'gun finish' under the hood, trunk, and bottom will save quite a bit as well...but I have a feeling he isn't going to go for that....looks like he wants those areas red.
Red itself is a little pricy. And you're not just leaving that out under the car port are you?