The pressure you need will depend on both the temperature and the desired carbonation level, so you'll need to use
a chart like this one to figure out what pressure to use.
As for the regulator and carb level, it's really personal preference. I use one regulator and a manifold to carb all of my beers to the same level. It might be nice to occasionally carb a beer higher or lower, but it's not worth the cost of the secondary regulators for me. Another option you didn't mention is a dual body regulator, which would save money over the 3 body, and still allow you to have 2 pressures/carb levels.
There are general style guidelines for carb levels based on style, but they're really just suggestions, and you should figure out what carb levels you like for yourself. Stouts, porters, and a lot of British beers often have fairly low carb levels like 1.8-2.2 vol. BMC and the majority of commercial beer is usually carbed med-high, typically ~2.7 vol. Belgians, saisons, and hefe's are often highly carbed, sometimes as high as 4.0 vol, which can be a huge PITA to serve through a draught system.
From what I've read here, most people seem happy with med carb levels for most beers, and I see a lot of people shooting for 2.4-2.6 vol. I keep my keezer at 40°F and shoot for 2.5 vol, which requires a 12 psi serving pressure.
