For a similar project, you might want to check out Mark Vigod's conversion of the Hangar 9 Twist 3D.

One thing the big electrics aren't at this time, is inexpensive. $700 for the LiPoly battery, motor, ESC, and support equipment sounds about right, though. You could do it cheaper, but you would be sacrificing performance. NiCd or NiMH batteries are cheaper, but heavier. Brushed motors in this size range are far and few between, so you wouldn't be saving much here.