Personally, I look for ones with matching serial numbers on the receiver, buttplate, and bolt. Then I just look for decent wood, no rust, etc. But then again, I'm not collecting. If you're looking for a collector gun, I suggest you check out 7.62x54r.net for WAY more information that you need.
In that case, you can pretty much buy one off the rack as cheap as $70. Bradis' has a ton of M44's and 91/30's in good condition for around $100, including sling and accessory kit.
As said above, matching numbers are a plus.
Make sure the rifling is good and that the crown is in good shape.
Have the head spacing checked before you shoot it.
Check for pitting and rust. Other than that, clean out all the cosmolene before you shoot it.