Wiki has the best overview...
https://en.wikipedia.org/wiki/Mosin_Nagant
And everything else you need to know can be found here...
https://7.62x54r.net
https://www.mosinnagant.net/
But in a nutshell, the 91/30 Izhevsk (Triangle with an arrow in it) is the more common than the Tula (star with arrow), round receiver covers are more common than hex ones; the M44s and M38s are carbine-sized versions, the former having a side-folding bayonet and are easier to find. You want to find matching numbers on the bolt, butt plate and floor plate (bottom of the permanently-attached magazine); and you want a shiny bore with good rifling. I stay away from rifles that have been counterbored, you can tell by dropping a 7.62x54R round down the end of the barrel. On non-counterbored rifles, the very end of the bullet (where it meets the casing) will be visible; in counterbored rifles it will drop down to the neck of the casing. I saw a nice M38 today but it swallowed half the bullet!
Lastly, you can find better prices online than at gun shows. I found a hexed 91/30 at this morning's show for $150 out the door, AIM Surplus has them for $89.95. Even with shipping it is 1/3 cheaper...
I think that covers it; if not someone let me know what I missed!
Oh, and this is why you want one!
Fucking huge fireballs!
Cheers! M2