Shopping list would be: no leaks, all lights work, electric brakes on both axles, axles not bent, ideally up to date on basic maintenance (hubs/bearings, brakes, breakaway battery, etc), empty weight no more than around 3600 lbs, 16 o.c. floors and walls (16 o.c. roof a plus), minimum of four d-rings, ramp door and side door in good shape, title in hand, etc. 

Budget is in the $3500-4500 range depending on condition.