First off, WELCOME TO MOTU COLLECTING! It is super fun, I hope you enjoy it!
I collected as a kid, lost everything to garage sales, collected against during college, sold it all to buy an engagement ring and bank the rest and, finally, began again a few years ago with an adult's wallet and display space! I am NO expert but I'll answer your questions the best I can. There are a LOT of talented, passionate people ehere so you are in the right place! Hopefully, they can chime in with better answers than mine!
1. Like anything, worth is determined by how much someone will pay. Check eBay 'SOLD' auctions for a quick reference. NOt what people are asking, but what stuff has actually sold for.
2. The European figures (the Euro Four) are worth the most - Lazer Light He-Man, Lazer Light Skeletor, Megator and Tytus. As for North American figures, some expensive loose figures are Scareglow, Ninjor, and other late series figures. Also, the 'spinners' (Rotar and Twistoid) are VERY expensive, often topping out at $150 for loose, complete figures and $300+ for MOC versions! The GOOD news is that many of the best figures from the early waves are cheap - He-Man, Skeletor, Beast Man, Merman, Man at Arms, etc. You might look into buying someone's collection to get a lot of these at once and then filling in rare figures here and there.
3. The black rubber band used to hold the figureds legs in often gets loose over the years, causing splayed legs. Watch for this when you buy figures. Some people can repair these, just be sure you know you are getting a 'repaired figure' or not. Paint wear varies, some color bleed is common on figures like Two Bad (his silver boot) and Roboto (his silver leg parts). The silver turns purple over the years on some of them. Also, Extendar's gold gloves often turn green because of the paint oxidizing or something. I'm no chemist. As for countries of origin, others know WAY more about that than I do. I don't really care about that, only major paint variations - like green/orange belt Merman, dark Grizzlor, painted back Tung Lashor, stuff like that.
4. He-Man himself I would think, though everyone has a personal favorite. Mine is Rio Blast followed by Beast Man. Trap Jaw is VERY popular overall.
Hope this helps at least a little! Good luck and, again, WELCOME!