This battery will fit in most buffer tubes:
http://www.hobbyking.com/hobbyking/s...arehouse_.html
If that proves too long for your tastes look at this one:
http://www.hobbyking.com/hobbyking/s...arehouse_.html
Generally, a width of about 21-22 mm will fit in most AEG buffer tubes. The 127mm length should allow you to set your stock at the second shortest length. I personally think this is a good compromise for a larger mAh battery. As has already been stated, 1200 mAh will not last a full day.
Ordering from Hobby King and shipping via FedEx is not prohibitively expensive if and only if you order from the USA warehouse (all the links I have provided above represent product in stock at the USA warehouse). When you browse hobby lobby make sure you are looking at product listing for the USA warehouse unless you really want to get hosed on shipping. I think that the US warehouse can still send batteries via USPS but I may be wrong (the last order I placed was almost 1 year ago).
Just do not under any circumstances accept any product shipped via UPS from outside of Canada because they have the most expensive customs brokers on the planet (can you say gouge boys and girls).
Go with a metal receiver. End of Story. A plastic receiver will break sooner than you would like. Heck, most of the monkey metal receivers will break faster than any reasonable person would expect! You have no idea what a beasting an AEG will take until you get out and take part in a few skirms or longer MilSims. Spend a little extra money now to avoid spending slightly less money again sooner rather than later (it costs a lot of money to be cheap).
And be extremely careful buying a used gun if you choose to go that route. Most used AEGs have 'only been gamed once' or 'only plinked in the basement' and have always been 'maintained by a professional gun doc' (there is no such thing as a pro gun doc - no regulated accreditation body exists). If you believe these claims then I have some beautiful cliff-side property to sell you. Caveat emptor.
And just because an AEG is advertised in the ASC classified does not guarantee the honesty of the seller. I personally know of one very thoroughly used gun currently listed which was described as having been 'only gamed once'. It should also be noted that most sellers here on ASC charge way too much for their used AEGs; I guess they aren't aware of the concept of depreciation.