Turbo BB vs dmax just depends on what you want for efficiency. Dmax will get great mileage, but they're expensive and a lot of work to swap. Turbo BB will get OK mileage if you can stay out of the throttle lol The dmax will make more torque at lower rpm, and sustain it much longer, but if you're not towing a lot the BB will do fine.
My rear mount turbo is just a gt45, but it is definitely small for the application. I think the heat loss from the turbo being 10' from the engine helps out with back pressure though, and I'm also only running 5 psi. As is back pressure is around 2:1, which is obviously higher than ideal, but could be worse. The turbo I'm running was mostly to get the system up and going, I'll upgrade it to a s475 or s480 eventually, but I need more injector and a walbro 450 first.
Currently I have a walbro 255 and 36 lb injectors ( flow ~42 at 58 psi), the injectors are maxed out at 5 psi the 255 is keeping pressure up so far but I am probably close to the max.
If you aren't planning to go much above 500 hp a walbro 255 should be enough, and a set of 36 or 42 lb injectors should work.
Stock injectors won't support much, so don't plan to use those, and I wouldn't trust the factory fuel pump personally.