Classic Army G36K is the way to go, it was my first gun.
You're going to find the ARES proprietary gearbox to cause you a lot of headaches, and you'll be waiting on parts to come in the mail, instead of having fun with your gun. The nice thing with a TM-compatible gearbox is that fixing and upgrading isn't a headache to get new parts in, and parts are often on-hand at your local airsoft store.
Proprietary internals are parts that only work with one specific gun/gearbox, i.e. the ARES G36 gearbox will only accept gears and other parts specifically made for the ARES G36.
There's some really awesome deals on CA G36Ks in the classifieds right now (I don't know why people aren't going for them...), If you get AV'd you shouldn't have a problem finding one.
G36 mags are a problem for finding suitable pouches, the other option is to get a STANAG Magwell conversion kit to run M4 magazines off of your future G36.