AEPs are great for CQB, their low power makes them super safe and you don't have to bother with a MED. Parts are not that rare, lots of upgrade parts exist, mainly from Laylax and PDI. Recently, a lot of 3D printed parts are being made available on shapeways from quite a few different people :
https://www.shapeways.com/marketplace/?tag=aep
Any AEP will work in CQB. With that said, quite a few other AEGs will also work great. The first really short AEGs to come to mind are the venerable MP5K, but more recently there's also the Echo 1 GAT and the Spectre RPD. Of course, there's tons of other choices, the AKS74U works great, as does the Sig 552 or the gazillion of short barreled armalites.