There are about a billion threads on Mosin Nagants on the forums.
Ultimately, if you want to scope it, ask yourself one question. Do I want to keep it in it's historical military configuration, or do I want to update, modernized and sporterize this rifle?
If the former is your answer, there are reproduction PU scopes and mounts. They're going to cost you more than what the rifle is worth by a long shot. Most Mosin Nagant M91/30 rifles sell between $85-$125. Last 'sniper' model I saw was priced for $600, and that was a reproduction. However, adding the 'official' PU scope and mount will require you to have the bolt handle turned down to clear the scope. I'm pretty sure this eliminates the use of stripper clips for reloading the magazine.
Now, if the latter is your answer, you have options. There are various scope mounts, some of which require permanent alterations to the rifle, others are not.
A scout mount is the least invasive option. It will replace the rear sight leaf (a non-permanent change), but requires an extended eye relief scope. My Mosin M-44 has a scout mount and 15.5" relief scope. You do not have to make any other alterations to the rifle. This is also the least expensive option.
You (or the gunsmith of your choice) can drill and tap the receiver for more modern rail mounts. This is a permanent alteration, will require the bolt handle turned down, and eliminates the use of stripper clips for reloading the magazine.
