Tokyo Marui AEGs are by far the best brand of AEG available. Whatever you do, don't buy Classic Army guns, they will only cause you problems.
To answer your questions:
1) Tokyo Marui is the best. I have a friend who has the M4A1, and he is very happy with it. The store I would recommend is called
DEN Trinity. They are a Hong Kong dealer, so your shipping costs will be more, but you will be very satisfied with their guns. They will even do upgrades for you. The one upgrade I have to recommend is to get a tight-bore barrel - it will definitely help your accuracy.
2) AEGs require minimal maintenace. They are not nearly as fickle as a GBB pistol. Just pop in a charged battery and go. This is of course true for a TM AEG....if you buy a Classic Army AEG you'll pop in the battery, shoot a few rounds, then return it to the shop because something broke again. Plus, since the guns are all quite similar, it is not too hard to upgrade and modify the guns for longer range, better accuracy, and even visual add-ons (laser sights, scopes, tracer units, type-R stickers, etc.).
3) Upgrade the inner barrel, maybe get a stronger spring (stock shoots about 285 fps, my upgraded gun gets 350 or so), scope, hi capacity mags, camoflague gear, radio, other war stuff from the army surplus store.....
You'll enjoy the AEG. You can get a full metal gun, but they are much heavier and don't really add much to the experience IMO. If it's your thing, then do it.....but still buy a Tokyo Marui gun. The internals are where it counts, and nothing can hold a candle to TM. Classic Army sells full metal body kits for most guns.