http://www.ebairsoft.com/scope-green-p-1231.html
If you don't get a lemon, it works wonder for GBBR. They looks POS compared to real M4. I have both. (M4 replica and M4s)
If you're going for Army look, M2, M3 and mainly M4 are the way to go. If you're going LE PRO is a good choice. If you don't care, get PRO.