Most of them are abit rattly mate. When i got mine i had a driveshaft on its way out, top mounts fuked, nocky exhuast, snapped spring, so it made it pretty noisy, sorted all these probs except the driveshaft, so its only abit nocky at times.
Why didnt you ask the previous owner what the switches where for ect, sounds like you have a bleed valve, but might not be working. Yes if it goes to half way on the turbo gauge its running stnadard boost, just geting into the red is about 14.
Theres a few guides on this website have a read about, temperature should sit just above the 2nd mark under normal driving conditions. Obviously if you stop it will get hotter, dont let it go to high tho!
5 oil pressure is all good