Quick update, decided on ditching the brushless drive for now and sticking with some hacked TZ85As and Brushed motors. Probably the easiest thing to do at the minute, no point messing about when I'm on a budget and don't really know what I'm doing in the first place...
Still having the brushless bar spinner though! Going have it as close to 4 kilos as possible, but need to figure out what I'll need to mount that all safely. Thinking of a steel box section frame above/below the bar, but I don't know if that'll be strong enough...