Here are a few ASC links that might help you out...
For my first AEG I researched who the good manufactures were/are and found who/where in Canada these parts were available and built my first gun... I did this to educate myself in the mechanical operation of AEG's and I am better for the experience.
Generally a TM AEG is awesome as a first piece - they are pricey but well worth it in regard to out of the box performance, longevity and upgrades available - it will last as long as you care for it.
Just take your time when looking at anything used or new and search the forums - there is a mass of knowledge here.