if you are patient, it can be easier to acquire the parts to build a full metal AEG over buying a full metal AEG or even buying a plastic AEG and upgrading and customizing.
two of my AEG's I bought used and upgraded them, then went full metal. neither contain more than 10% of the original gun. some parts broke front trying to make them fit with new bits, some are just sitting in a box.
Getting the metal bodies though can be a time consuming task and requires lots of patience. BUT, there are the odd times they pop up in the classifieds as long as you're quick they can be yours!