You don't need anything except the issue sights but some folks do enjoy home gunsmithing the Mosins. No harm in it but the issue sights are pretty good once you have them dialed in, much better in my estimation than any issue Mauser sight. Much easier to aquire a good picture. I have not scoped any of mine so I have no first hand experience there. Asking questions on Surplus Rifle dot com will get you a pretty good answer to just about anything.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer