Hey Tony,
Heres my opinion but keep in mind, everyone does it a little different as airbrushing is a learn as you do process and adapt to what works best for them.
Personally I like waterbased paint (createx/faskolor/autoair) these are readily available and simply to clean out in between and after use (you will most likely require thinner (auto air or Jon at RCS sells some as well, he also stocks createx and his own brand of paint).
Body shells I normally mark up first and then cut out after, but have done both ways and find them both fine, I prefer cut buggy bodies but uncut touring cars as the sides flap when spraying once cut.
PSI is very questionable as it depends greatly on the paint and how it is thinned etc, I tend to find with most opaques I spray at around 25-30, and less for detailed work of around 10 and under (shadows etc).
White however seems to like to be sprayed at a higher pressure so sometimes depending on the area I will spray around 50psi.
Hope all that helps and just in case RCS liquid mask is the way to go unless youve got a cutter
best of luck.