heavydluxe,
Kinda funny, that's exactly where I started... Marlin 795, then I got a 91/30 to play with while I was building an AR.
Got my 91/30 at AIM Surplus for $80, just took a bit of cleaning and it came out great. Bore is near perfect, everything matches, functions great, and no sticky bolt problems.
If you're getting your first Mosin, try to find one with at least a matching bolt- it eliminates any potential headspace problems.
Hex vs. Round receiver is purely cosmetic, and of more interest to collectors. Generally hex look better, but they don't work any different in the field. (Hex is also a misnomer; the top of the receiver is a half octagon and the bottom is round.)
If you do find one at a shop or at a gun show, check out the bore and make sure it's pretty. Maybe bullet test the muzzle? (Put a round tip-first into the muzzle and see how much stays sticking out- tells you how much rifling is left near the muzzle.)
They're great guns and I can guarantee you'll love it.
One more thing...
All surplus ammo is corrosive. Just wash out the bore and bolt head with soap and hot water or 50/50 ammonia and water after you shoot. Modern production ammo is non-corrosive and there's nothing to worry about.
Have fun.