RE: Military surplus rifles
K-31 would be my top choice for the money- I currently have 4 of them.
Mosin nagant 91/30- If you buy a pre-war model (especially if made by Tula)that is in "excellent+" condition, you'll get your money's worth out of the rifle-it will probably be accurate and well made. Anything made during the war will be a crapshoot- they slapped millions of them together in a hurry and didn't pay attention to the details very closely. A nice thing about 91/30s is that they balance almost perfectly and ammo is very cheap and plentiful for them.
Enfields- AIM surplus is selling Ishapore 2A enfields right now. The neat thing about them is that they are chambered in .308. Of the enfields that I own, my favorite is an Ishapore grenade launching SMLE.
Mausers- anyting made by the Czechs, Swedes, or Germans is worth owning. There are lots of Yugos out there too, some are pretty nice, lots of them are worn out, and I haven't had much luck with them- The quality of these rifles is very poor compared to the others.
I wouldn't 'work' on them in any way other than cleaning them up. A $100 K-31 might be worth $500 5 years from now. If you "work" on it by apply tru-oil or polyurethane on the stock and rebluing it, it will probablynever be worth more than what you paid for it, at least in your lifetime.