A Mosin Nagant right now for under $300 is pretty good. Six months ago you could get a 'new' one out of the crate for around $120, and twenty years ago ou could get them for less then $100. I bought one for my Dad back in '93 for something like $75. They shoot the 7.62x54R, which is a rimmed cartridge, thus the R. It sounds like the one your looking at is 'straight out of the crate', so the bore is probably full of rust inhibitor. Some good degreaser, brake clean, etc should clear the bore and make the old girl look as good as new. Well, maybe not new exactly, but if it's like Dad's a good cleaning, and a light sanding and refinishing of the stock, will turn into a really beautiful rifle, with a bit of character to show for it's age.
I know for a fact they are accurate, and powerful out to 100 yards and beyond, with just the iron sights. I say that because I've put a FMJ round completely through a tree that was nearly 12 inches in diameter at a distance just over 100 yards with Dad's. I believe the muzzle energy on a typical round is anywhere from 2500 ftlbs to 3000 frlbs plus. In other words enoug take down power for anything you might want to shoot.
One problem they do have is that they are a PITA to mount a scope on, and really need a gunsmith to drill and tap the receiver to really get it done right. Even then something else that is typically done when a scope is installed is to bend the handle on the bolt down vs being straight to allow clearance for your hand to make cycling it a bit easier. It's either that or mount the scope off to one side, which would be my prefference as a scop sitting on top of the reciever blocks the bolt preventing you from using a stripper clip to load it.
They are truely amazing rifles but they do kick like a mule, again I can state that from experience. My biggest problem with them is the length of pull is to short, so, unless your a small fellow you need to add plenty of extra lenght to the stock to make it fit. Beyond that all I can say is I am currently kicking myself in the butt for not buying a nice, sporterized M44 a few months back for $175 when I saw it in a pawn shop. Of course I'm kicking myself even harder for not buying the Norinko SKS paratooper, with a factory 20 round star mag, in another pawn shop, again for $175 a year back. Right now a gun like that is selling for nearly $1000........I guess hind sight is 20/20