Even if you will buy +100% rep boosters, it will be slow and booring, so just level them up. As for new heroes, unfortunatly, there is no way for fast farm. Buuut, to be honest, i completed Homestead, Rovers and Prison by default while just playing story campaign without farming dungeons (by lvl 65 or so).
On elite difficulty i am getting like +1200 rep for completing single bounty quest, so it would take just 2-3 hors to max it out if i really would want it.
