I suggest taking a look at some of the sites out there that have a wealth of information. This is a good one: 7.62x54r.net
The basic gist is that there are two "countries" that you can find rifles from - Russia and Finland. The Finnish mosin nagants are actually Russian rifles that have been arsenal re-worked to higher standards, so they are more accurate and thus more expensive.
You will find two receiver types, round and hexagonal - some price Hex receivers higher than round (they were usually produced earlier than the round receivers). This is because the rifles produced during wartime (round receiver) may have been rushed through production, resulting in lower quality.
Ammunition is cheap- $80 to $100 for 440 rounds of corrosive surplus ammunition.
You could take the rifle and try to do all sorts of things to it to "improve" it, but like those that posted above said it kind of is what it is. It's plenty accurate for most shooters, it can take a huge beating and you won't care, and it is fun to have something so cheap that goes big boom.