1. I run my pistols on 100 psi and they shoot about ~335 fps on 0.20. I assume your bolt is heavier than any of my pistol slides so in theory you may need higher pressure to achieve the same output.
I'd go the other way around actually and would start from 75-80 psi because lof of people run their MP7/MP9's on those pressures for indoor which still gives them average 270-280 which is absolutely enough for shooting people close range. But my pistol on 80 psi is sluggish (alu slide is too heavy for it) so it's gonna be trial and error in your case as well.
I don't think I'd go above green gas pressure (average is 110-120 psi?) especially not in summer. Basically the more pressure you put on your internals the more they will wear. An HPA engine is a completely different animal compared to a gas gun but going above 120 psi is not recommended even on them and not needed anyway.
2. US is standard but EU fitting does exist. They aren't compatible with each other.
3. I woudn't buy cheap regulator but a Balystik HPR800C or something similar should be enough. Also, that new Airtac regulator can't be bad either.