1987 supra turbo help

well ok i have a 1987 supra turbo with a 7mge and i was wondering since it is right now pushing out 6 psi or so wat is the max psi i can crank it up to with it damaging anything, currently i have HKS 1.2mm metal headgasket and ARP headstuds were put in when the engine was swapped in. There's also a 3inch stainless steel turbo back downpipe and a 3 inch test pipe mated to a 2.5 inch catback

10-12. Make sure you upgrade the fuel system.

what should i do with it

At the very least you will need a piggy back fuel controler and a wide band O2 sensor to tune it.

how much you think that cost

It is sounding to me like you should not play with these thigs yourself. You should read up on how turbo charged engines work, the things commonly used with aftermarket systems, and then make some informed decisions.