I have 6 of them right now, from my experience, I offer the following guidelines for buying a Mosin:

1: The carbines aren't all that accurate. The M38s were built during wartime and tend tohave rough barrels. The M44s made during wartime have the same problem. If you want to get a carbine version, buy a post-war dated M44.
2: Finnish Mosin Nagants are by the far the best and most accurate Mosins out there- unfortunately you won't touch one for anything near $100.
3: 91/30s can vary from really good to crap. The best ones were built before the Soviet Union was invaded. Rifles dated 1940 or earlier are going to be the best. Rifles dated 1941 and later are going to be roughly machined, poorly fitted, will have varying degrees of accuracy, and won't likely be zero'd correctly- they usually shoot high, which is kind of difficult to fix. Any rifle made at the Tula factory is going to be a cut above those made at the Iveshk factory, though they are rarer (the Tula built rifles will have a large star on the barrel shank, Ishevsk rifles have a triangle with an arrow in it on the barrelshank.) FWIW, my best shooting M-N is a 1938 Ishevsk 91/30.
4:A lot of the recent and commonly seen M-Ns for sale are 'rearsenalled to excellent condition.' Some of these are like-new rifles, others have just had the stock replaced and still have a bad bore. Inspect the bore before buying- it should be bright and shiny.
5: Make sure the serial number on the bolt matches the serial number on the barrel shank- if it doesn't match- DO NOT BUY IT. Note- due to recent BATFE rules, the importer needs to assign a serial number to each rifle- it will be obvious as to which is an importer engraving and which is the original factory serial.
6: Don't buy on impulse. There were only about 30 million M-Ns made- most of them of common models. Hold out for one with a perfect bore that is a pre-war 91/30 or a post-war M44.
7. Counterbored barrels- if you don't see any rifling at the end of the barrel, its been counterbored to extend the life of the barrel. They can shoot good, but for the most part won't shoot as good as a rifle with a new barrel.

Wolf, Igman, and Sellier&Bellot both make hunting ammunition for 7.62x54r- for around $14/box. You can also buy surplus ammo in 300-800 rd 'spam cans' for about 10 cents a round or less.