I would take their advice, I have been building a custome gun for about 6 months

and its an armalite model, it seems like a relatively easy thing but there is the sourcing of all the parts, having them shipped and then putting them together. All good until you discover that you need motor base plate screws, pistol grip mounting screws, need to rewire it from a front config to a rear one and you've torn the bucking sleeve while installing the barrel.
Then there is the whole gear box thing, shimming, aligning gears properly, reinserting the anti reversal latch spring in the correct way and tucking in that pinched wire inside the gearbox and opposed to inbetween the two halves.
I do not know you level of experience with AEGS but building you own is likely going to cost you more than buying one and create more hassle than its worth unless you are the patient type, I killed most of my patients...
Now to flip the coin, with the gun I am building I know its one of a kind out there, looks great in the condition it is now and really looking forward to finishing the project, I have a list of theose screws I need and will be going out to buy them today. A team mate came by so i could work on his gun and now he wants to do the same with a gun he has, no greater flattery than imitation they say. I guess Frankenstein is beautiful to someone
Good luck if you decide to carry on building your own, its going to be a journey of discoveries and some good hands on AEG experience. Should you screw it all up dont't come on the forums and complain, we told you so :P
Mud