The amount of power per psi also depends on the altitude (density of air) where you are. I'm going to be doing a Twin turbo kit. On3 performance as far as I know its a good kit. very complete. Check out theturboforums.com they have alot of people running that set up there. A single kit will make enough power and it'll be easier to install but it'll take a little time to spool. If you were to go with a twin kit yo don't need .35s I don't even know where you would get them. I'm going to be running a set of .50 Compressor .63 Turbine housings. They should spool up faster than the single and still make plenty of power. Money wise depending on which turbo kit you go with and what the goals are could be anywhere from 3-5K. 3K with the cheaper kit/smaller kits 5 with a medium to big kits. That could include fuel system requirements.
Fuel pump (I got the 255 fuel pump), regulator (going to but a aeromotive with the 1:1 Boost/vacuum), injectors (42lbs for me), stock fuel rails for me. You could buy some Trick Flow or BBk. etc...
Air- I hope you dont plan on keeping the stock intake. I'm using Track Heat Trick Flow intake, 75mm TB, I think its an 80 MAF matched to the 42's. Stage I Trick Flow cam (I got it for cheap, but a custom would be better) and stock heads with Trick Flow springs.
That with the kit that I got that includes, Turbos, Cold side piping, Manifolds, WGs, BOVs, oil lines and fittings. Don't forget to drill the oil pan for oil return. That pretty much sums it up. If you have any other questions (besides how it runs and how much power im making *coughs* not done yet *coughs*) you can send me a PM. :bigthumbsup
I'm sure other guys here will point out something I forgot. Thanks guys