Beatdown is probably your best bet for ease of use and affordability - however one of the best beatdown decks - Little Brotherhood - is actually quite expensive.
A deck that is actually quite cheap and can be effective is Xmen/Brotherhood based - soemthing like:
I'm bad with the second names, so
4 x Nightcrawler (2 drop)
4 x Toad
4 x Wolverine (3 drop)
4 x Rogue (3 drop)
2 x Rogue (6 drop)
4 x Mystique (3 drop)
4 x Arcade (this guy rocks on turn 3 with equip
4 x Dual Sidearms
2 x Jetpack/BorrowedBlade
2 x Advanced Hardware
4 x Marvel Team Up
4 x The New Brotherhood
4 x Flying Kick
2 x Acrobatic Dodge
All of the above are common/uncommon and a lot of these are available in the Marvel starter precon.
Some rares that would suppliment the deck nicely but you can get by w/o at the beginning
Cerebro
Nightcrawler Fuzzy Elf
and some of the other rare characters
It's partial only and obviously completely untested, but a deck like this nearly beat me at the PCQ (and should have if the guy knew how to play better). Turn 3 8/6 Wolverines are pretty hard for most decks to handle
If you "decide" on a deck, we're more than happy to donate the commons/uncommons to your cause - think of it as a VSParadise service for their staff