Oh ye of little faith! M20s are stout motors if they are maintained half decent. The only thing that starts to happen to them when the mileage gets high is they start oozing because mechanical seals don’t last forever. Adding boost shouldn’t change how those seals are working though…unless you forget to vent your oil vapour recirculation tube to atmosphere and start pressurizing your crankcase lol.
Starting with a motor that is in good running condition is important, and by the sounds of it you’ve been freshening up yours a bit, which is good. It’s too bad you already did the timing belt because when you had it off I would have recommended pulling the head, getting it decked, replacing all the seals, and reinstalling it with some ARP studs. However, if your valve stem seals are still good (i.e. when you engine brake at high rpm your car doesn’t put up a James Bond smoke screen, then you can just replace the head bolts one at a time with ARP studs and you should be ok….upgrading to some ARP studs should definitely be on your to-do list though if you’re going with boost.
If it makes you feel any better, my turbo E30 was a beater from day one (also around 300k) and I’ve had it boosted for about 15,000km from anywhere between 10-15psi and the engine is running the same as it was before it was boosted.