How much boost for 500whp?

Pretty much anytime you lower compression ratios, you lose power. If you were to build the motor with lower compression pistons and not have the turbo kit on it, you would feel a noticable difference in power loss. The benefit is that you will be able to run more boost and thus increasing power with the addition of the turbo kit.

You really need to understand the fact that NOTHING is completely safe with turboing any factory n/a motor. There is no set amount of boost or horsepower level that says “if you run this amount or under your motor won’t blow up.” All of the “safeness” lies in the proper tune of air/fuel ratios.

Not to sound brash, but I think you are trying to take on way too much at one time here.

If I were you, I would start with getting a decent turbo kit, a decent engine management system, injectors, a fuel pump, a lower compression headgasket, headstuds, a good clutch, and a full 3" exhaust. Then take it for a good dyno tune. See where that takes you first.

The built motor can come later, or you could even buy a spare block to start piecing together a built motor to throw in at a later time with the stock one craps out or maxes out on you.