As per Impulses reply - Cyma MP5 is a classic choice. I've played a lot indoors and out with mine. They are solid. The only slight downside is the sliding hop unit can sometimes seem to self adjust - a known problem, and very quickly sorted out with a small O'ring placed against the slider to add more friction. Mine is an A3 version - CM041J Blue Edition - and has been cracking.
I also have a very budget (£110) Specna SA-FX01 FLEX - ar9 style gun. Its super light (as its entirely plastic), super cheap, and phenomenally, comes with a programmable ETU. It's not bad at all, although the hi-cap mags seem to frequently jam with BBs caught in the mechanism - requiring the mags to be stripped to free them up. Mid caps are available.
The other gun that could make it onto your list is the Double Eagle UTR45 (M917). A little heavier than the Specna mentioned above (but lighter than the MP5), but feels nice and solid despite being fully plastic, and again comes with a programmable ETU. Generally a well regarded gun. One of the downsides are that the mags are huge - albeit that is a key part of the .45 calibre aesthetic. I seem to recall that the stock motor that comes with them is a little lacklustre.
All three of the above have quick change spring systems for their gear boxes - particularly useful for downgrading the power for CQB purposes. If you ever want to start investing more money over time in them, all three of the above are readily upgradable with new inner barrels, hop buckings, and motors if you ever feel the inclination, and the AR9 and UTR also have the advantage of using M4 style hop chambers too, with a plethora of after-market options (albeit that doesn't necessarily translate to better).
Of course, if you want something short and stubby, and real world aesthetics aren't too much of a hang up, then the world is your oyster for super short M4/AR 15 style blasters with tiny out barrels and PDW stocks, but personally I’m not overly keen on the look.
