For heel down, I wouldn't suggest a longboard. You'd require more effort to get the pedal moving, due to the fulcrum point being further back.
Pearl Eliminators, DW9000/5000, Iron Cobra or maybe the Mapex Falcon get my vote.
Play heel down most of the time and I've been using Iron Cobra's for 20 years. Love 'em!
Up to $300 budget - I think the DW's, and the IC's are out of the picture. Not sure of the others.
....You have to buy used to get under the $300 threshold