For an AEG a scope is fairly pointless, since for these events you won't be going over 400 FPS. I'd say if you want a full length m14 either run the iron sights, or get the AGM, as far as the interwebs can tell me(I did an amount of research on low budget M14s before selecting my M4) the internal qualities are fairly the same, and the agm comes with the scope mount and an extra mag for cheaper, you can get it at evike. The only major difference is a slightly different(not worse or better, just different) scope mount, and no bipod rail, although you can get a bipod mount.
I'd like to point out before all the trolls say don't order evike, that it's an airsoft website like anyone else, they make mistakes like everyone else, but there's a high chance they won't.
For optics, if you really want them, one of the various 3-9X scopes will do fine, the 3X for fighting and the higher magnifications for looking really hard at things. otherwise I'd say get a red dot, one of the aimpoints with the 2x screw-on attachment would work nicely.
http://www.airsoftpost.com/product_info ... s_id=27162
http://www.airsoftpost.com/product_info ... s_id=26684
forgive my ignorance, it happens to everyone.