As much as the denter is responsible for giving you straight, aligning lines and blemish-free sides, the painter is even more responsible to do justice to the bodywork and bring out the finish. The painter is the last person you want to take advice from. Yes, the painter would be able to highlight the areas that need to be worked on by the denter but once he has done that and the denter has fixed the issues, its down to the painter to properly finish the car by excessive sanding (block sanding) and nominal use of filler. Basically, once the base is done, before laying down the actual paint, ask the painter to lay down a couple of coats of semi gloss black (definitely not matte black). That would highlight any imperfections that need to be worked on. Once they are taken care of, then only have it painted. While its painted in semi gloss black base coat, you would also be able to judge what it could look like when painted black. If you feel that the sides arent perfect or the finish wont come out as good as you want it to, you can then decide to go for a lighter shade color.
Also, for the sides to be perfectly straight, the painter needs to use a large size block with sandpaper on it (block sanding) to do the sanding. That would highlight the pits and the bulges and would tell the painter what area needs to be filled and what area needs to be sanded. Make sure that he sands all body panels as one, installed and adjusted to their final positions, to prevent the panels from bulging from the corners. Another trick to judge the sides is to place a lit tube light on the side of the car and see its reflection in the semigloss painted surface from all angles. The straight line of the tube light should reflect as a straight light on the body work. If it doesnt, you'll know that it needs work.
About the 3SFE, you've made a good decision. It can give you that kind of mileage if the wiring is done properly and all the mechanical work is done properly as well. Of course it comes down to the driving style as well. If you keep the revs high, it'll consume fuel. That said, it has enough torque for you to keep the car at low rpms while driving it, giving you good mileage with occasional bursts of speed.