You mention two different purposes. Collecting guns and using them is a bit of a dichotomy unless you're Ian and Karl or C&Rsenal and do "historic shooting" for the purpose of research.
The first thing you should do is figure out what you really intend to do with the Mosin. If you want something you can shoot, just find the cheapest M91/30 possible that isn't beat to crap. That being said, they're actually not very practical guns to shoot. You can't really compete in anything with them except in special WW2-oriented matches or if you get a Finnish Mosin and shoot say High Power with one. You can hunt with one, but they're not as effective as a used Savage 110 or Remington 700 with an econo-grade 3x9-40. The stripper clips suck. Some people brag about special brass or Finnish or Chinese stripper clips, but the stripper clips generally suck. If you want to collect guns and not really use them for anything, just buy whatever Mosin variant interests you.
The second thing you should do is to figure out all the variations of Mosin:
7.62x54r.net
Finally, set yourself a realistic price point. There are really quite rare Mosins out there, like say an original M91 dragoon or a French-made Mosin or a Mosin from the very early stages of production with the metal faux pistol grip, that command a higher price due to basic supply/demand issues. Then there's Finnish Mosins, again, which command a higher premium. Carbines also cost more because again, less supply and more demand (despite being relatively common guns).
If all you want is a gun to shoot at a 100 yard rifle range every other Saturday, there's a huge amount of other options like Carcanos, Arisakas, French rifles, or if you're not in Mass or CT, ARs. Pretty sure even in Mass, you can get an inexpensive SKS for under $500 still, especially if you get a C&R license.