15 psi is perfect for a completely stock dsm. However, the stock blow off valve leaks at about 11 psi, so u cant go past that anyways.

IMO heres what u should buy first:

boost guage(absolutley needed for changing boost)
blow off valve
manual boost controller
some kind of intake, or performance filter
Upper intercooler pipes

then u can go onto the more expensive things like exhaust and fuel mods to support whatever size turbo u eventually plan on getting.