Bet bang for the buck is a 80's or 90's 2S aircooled Mini MX machine
tons of FACTORY parts available and aftermarket as well.
Don't be fooled by "new" tech......... there are ooooooooold bikes out here that PHLY

My local mini frEEk is buying up some older 2S machines for pimping at low dollars, i mean less than $200 a bike.......... another carefully spent $300, some expert massaging and they riiiiiiiiiip
Best thing about them is the parts availability, ease of custom or adaptation, and Japanese reliability.
Easy to have multiple wheel sets and ride it MiniMX, ICE, Pavement, SUMO.......