Lots of questions, and that's good. I have been shooting a couple years now, and am still very much a novice. As long as you keep asking questions, you will keep building knowledge. Eventually you will be helping the new people.

Anyway, to answer your questions, the 15-22 is polymer, and you should not worry about it. As long as you take care of it with proper maintenance, it should last you forever. That said, if anything breaks it is all proprietary and there are very few aftermarket parts specifically for the 15-22 and will have to be sent to Smith and Wesson for service.

However, most AR15 accessories will fit when you want to enhance your rifle, including triggers, optics, pistol grips, and handguards if you don't mind putting a little work into it.

Lots of people use the 15-22 for small varmit hunting. You won't kill a deer, but small rodents you should be fine.

22 long rifle is a great choice for a first caliber. It's cheap so you can practice a lot, and has minimal recoil. Your fiance will probably like your 15-22 so much that she ends up getting her own. My 15-22 is my girlfriend's favorite of my guns (out of a 9mm Sig pistol, 5.56 AR15, and the 15-22).

I can't tell you if it is better than a 10/22 as I never owned one, but I can tell you that if you buy one, you won't regret it.

Good luck