The Mosins are fine rifles, not top of the line but excellent serviceable rifles.
The most important thing to look for is the barrel. Take a bore light and make sure you get a clean barrel.
The biggest problem with the Mosin is people don't take the time nor put in the effort to learn to shoot them. They can be pretty dern accurate rifles with the existing sights.
If you get one (and I think everyone should have one), find a CMP GSM Clinic in your area (up coming clinics are listed on the CMP Websites, more will be added to the list when the weather starts to warm up). You'll learn to load them which is a bit tricky at first.
Yes most shoot high, that's an easy fix, I've posted the procedure several times on this and other forms so wont go into it hear. I'm talking about fixing the existing sights keeping the rifle within the CMP Vintage Military Rifle Rules.
I'm not a fan of cheap surplus ammo in my guns but a lot of people shoot it. I reload with quality components and mine is quite accurate. If you don't reload Winchester has ammo that is pretty good, accurate, and can be used for hunting.
Some people "bubba them", not my thing but for the price one can get more then one, modify one and keep on "as is" to keep the historical value and with in the CMP GSM rules.
Try CMP GSM Matches, lot of fun, and a good place to learn to shoot your rifles (not just the Mosins).
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071