I think between coats 400 would be fine. 600 would work as well. You can get the PC from Lowes ($109). Take a look at the Hitachi (you can get it for $110 shipped from CoastalTool.com that is where I got mine). Having both the PC and the Hitachi rotary I can say the following:
1) The PC will bog down if you apply some good pressure on it.
2) The Hitachi can do the work of the PC a lot faster.
3) As long as you keep the Hitachi on lower speeds (1-3) and constantly move it will not burn/damage the paint (this is for polishing and more for a 2 stage paint job base+ clear which is not the case with our system here ...well some people are doing the tremclad and then top it off with clear coat).
4) There are scratches/defects that the Hitachi will get out and the PC will fail (all things being equal both using same pad and polish).

If I had to do it again I would just get the Hitachi (once you use the 11 amp beast you cannot go back to the 3+ amps of the PC).

I don't mean to bash the PC. It is great but its greatest strength is its greatest weakness. The fact that is is random orbit makes it so anybody can use it without fearing of damaging the paint (this also robs the power and makes things take longer). The rotary is not something people should be afraid of. I've learned to use it and everyone can (just start with the lowest speed and always keep it moving). I'm going to do a video and try to post it eventually.