The way I understand it, the big benefit of brushless motors is that they can accept a wide variety of voltages. You can start out at 36V, and the popular 9C brushless hub has been run at 72V-111V successfully.
That being said, if you know ahead of time exactly what you want to do (certain there will be no future voltage changes), a brushed motor is not a horrible choice, and sometimes a brushed system makes the controller much more affordable. Efficiency is a wonderful thing, it's the gift that keeps giving. However, non-hub systems allow a lot of options, and the higher efficiencies of a non-hub system could off-set some of the in-efficiencies of brushed components.
That being said, avoid high amps through the brushes. How high?...every system is different, and every hill is different. Add a CycleAnalyst, so...at least then when you fry something, you'll know where the cliff is for next time.
Grampa Chas (over at VisforVoltage) used a brushed 24V motor @ 36V, driving a 3-speed IGH on a delta trike that was geared for 17-MPH top speed. It was a great hill-climber, according to him.