Several things.
Mr. Caliber--OK, starting back several posts--The single-shot you mention: It would do, it looks nice, go ahead & buy it if it calls your name. It may or may not be The Rifle For You, but you won't know that for sure until you try it, and the same thing can be said of any other rifle you might purchase. Nice thing abt rifles is that if you decide to resell you can take it to a gun show and hawk it (Don't sell to a dealer--walk up & down the aisles with it on yr shoulder and a sign, "Rifle for sale" or some such. Guys will ask, guys will handle it, you can dicker, and, no middle man--you sell for what the dealer would have sold it for!
) You could buy a milsurp for less, however.
Jesse's comment abt an assault rifle is right on--if it won't go full-automatic firing (like a machine gun) it is NOT an assault rifle. No matter what Sarah Brady says.
The Mosin-Nagants are cheap, and so is the milsurp ammo, though much of it is corrosive. (Just means you have to wash yr bbl. w/hot soapy water right away after shooting, before going home to clean normally. Not much of a big deal. But you need to know to do it.) 7.62x54R is clearly a big-game cartridge, and it has a recoil to match. Mosins have a reputation for kicking particularly vigorously, although as with any rifle, (a) that depends on how you hold it to yr shoulder, (b) it can be got used to, (c) it affects some folks more than others, and (d) when shooting at game, you will never feel it. The carbine version will kick more so than the full-length rifle, just because the rifle weighs more--Newton's Laws of Motion apply
here.
7.62x39 and 7.62x54R are NOT the same cartridge. The '39 is for SKS's and AK-47's and the like--it is about equivalent to a rimless .30-30 cartridge ballistically. The '54 is for the Mosin-Nagants and some other Russian Army rifles--they still use it for their ultra-modern sniper rifles. And as has been said, the '54 is very much like our .30-'06 ballistically.
Cheapest 7.62x39 is yr own handloads. Buying it cheap requires shopping around. It's in short supply currently, but that is supposed to be a temporary problem.
And lastly, an AR-15 will set you back at least 5 bills or so to get into the game, and they go up, up, up in price from there. The .223 ammo is cheap, but that initial investment...ooh la la.