20 psi on a stock T28?
Kicker is, he's planning on doing the headgasket anyways. He's gonna take the damn cams/head off either way. If he can change a head gasket with help from a couple of friends, I dont see how he can't put drop in cams with it.
Personally, I would go stock HG, ARP head studs, double rocker arm guides, fresh oil pump, water pump, Nismo thermo, Z32 Maf, clutch fan/shroud. For tuning, all up to you. People aren't a big fan of PFCs since they're. They're still good but there are better options nowadays for as much as it would cost you to get a new PFC, commander, dataloggit and tuning cost. You can do more with AEM.
Rom tuning, this is a cheaper alternative to running a standalone. I'm not a big fan of it but since you're in FL, JBB pretty much hit it on the head. Martin can rom tune your ECU on the dyno, that's the only way I'd run a rom tune. Mail order rom tunes...Never again.
From what folks who's done the double rocker arm guides, you can forgo rocker arm stoppers, but still wouldnt hurt to have them. If you're planning on getting fresh bronze valve guides and you plan on drifting (bouncing off the rev limiter), wouldnt hurt to run some stiffer valve springs and titanium retainers since you're already there.
Turbo wise, 300hp or 300whp? If whp, SpecR T28 may not be enough to get you there. Like KingTalon already said, GT28RS, maybe even a .64 trim GT2871R. Either way, 550cc wont be enough to support either turbo. I'd go atleast 650cc or even some 740cc. Wouldnt hurt nothing to have larger injectors since the car will be tuned for them, always better to have more than less. Plus you wont have to worry about getting another set of injectors when/if you decide you want more power later down the road. Also, if you decide to go larger turbo than the T28, get with CodyAce and get your stock manifold extrude honed/go external wastegate.
Only other thing you didnt take into consideration: Clutch and fuel pump. Run atleast an Exedy Stage 3 (3 puck) or ACT clutch (stay the fuck away from SPEC) and don't go Walbro if you want a reliable pump. Bosch 044, maybe even a brand new OEM Z32 TT pump.