I own a 1943 Ivhevsk 91/30 Mosin Nagant. I got it for $150. It's a good gun, fun to shoot, and has a good amount of recoil.

Depending on where you purchase it, it should come with a sling, bayonet, ammo pouch, toolkit, and an oil can. Ask if it does.
The big thing is cleaning. These guns were packed with cosmoline to preserver them and when you buy this rifle, guess what, all the cosmoline is still usually inside it. Your going to have to clean it out. Heat melts the cosmoline. My friend cleaned out my Mosin for me, but I cleaned out my Garand. What I did was take the entire thing apart (Mosins are very simple to take apart and put back together) and put the pieces into a tin container and dumpped boiling water over them. Let them sit and the cosmoline should come off easier. Don't forget the barrel aswell.
As for ammo, I can get military surplus 7.62x54r ammo here for around $8-$9 for 20 rounds. And as most of the guys said here, these rounds are corrosive meaning you have to clean your gun right after shooting it. That being said, I recommend getting a cleaning kit. The Hoppes#9 Rifle cleaning kit is pretty good. It comes with lubricant that helps protects against rust, Solvent that helps get rid of rust and also gets rid of the corrosive material, patches (not many so pick up an extra pack of cleaning patches), cleaning rod, and 3 cleaning rod end knobs.