Depends on what shape the parts are in. On aluminum with heavy scratches or gouges I sand out starting around 180 or 220 grit and go up to 600 wet. Normally best to go up in grit number 100 or 120 each step. After 600 wet or if the part is in decent shape I'll go with a spiral sewn and black compound, some use brown compound next, I usually skip this one and go with white compound on a loose cotton. I finish with a red or blue on loose cotton. This is in general. All parts are a little different due to material grade and how they were processed. After those differences you'll have to find your own feel for doing polishing because what works for me or others may not work for you the same way.
The wheel size will depend on the size and RPM of the tool you get. Bigger is not always better. It won't do any good to get a big grinder with a large wheel if you cannot hold it and control what you are doing, not to mention your arms will either get very large or fall off usually the latter. You'll need a different wheel for each color compound.
Have you read the free buff book Caswell has? It will help to understand the process as well as the importance of surface speed and the relation of RPM's to wheel size.
http://www.caswellplating.com/buffs/buffman.htm
Polishing isn't hard after you've been doing it for awhile. There maybe some bumps in the road getting there though. It's like most things, it's in the details.
SS