How well does your rifle shoot with the irons? If it doesn't shoot reasonably well with irons, scoping it will not make it shoot any better. A scope only lets you see the target better.
Secondly, how much money do you want to spend? Like Aragorn243 says, you'll need a long eye relief scope if you opt to go with removing the rear sight. (That also means you're done for the day if something happens to the scope or mount.). However, you'll also need rings.
Have a look at this. It's a repro of the original Mosin sniper scope and mount. $208.00 is probably less than a long eye relief scope by itself.
Mosin Nagant Series