I'll give you a hint before ORCA does the wise thing and locks this thread: You won't get a useful true support weapon for less than around $700-ish. True, you could just get a Maruzen MP5-K, 50 or so standard mags, and a few cans of green gas (trust me, you'll need them), and you'd be good to go, to some extent. But that's not really a support weapon.
You could get a TM mp5k, upgrade the moter. But then you would have to upgrade the battery and put it on the outside. You would also want the spring upgraded to 400 or u wont hit Shit . Meh so ur prob at 500 bucks there. Then spring another 100 for a double drum. 600 bucks. Meh, i would just then go with an AEX custom gun. Such as the Muskrat. But that has less range then most heavy machine guns, so blah.