Hey EricH. Thanks for your insight. Sounds like I should be OK on the current draw as long as I have a decent motor driver. I'd definitely spring for the Victor or Jaguar controllers if I had the money, but unfortunately their costs are driven a little high by the inclusion of the motor control components.
I have found a few (theoretically) suitable made-in-China motor drivers on eBay -
e.g.
http://www.ebay.com/itm/High-Current-DC-Motor-Driver-Module-MOSFET-control-for-Robot-Arduino-Projects-/220984305430?pt=LH_DefaultDomain_0&hash=item3373b0 e716#ht_7302wt_1398
http://www.ebay.com/itm/High-Current-Dual-Motor-Driver-Module-Board-Heat-Sink-PWM-Signal-NEW-/221043792022?pt=LH_DefaultDomain_0&hash=item33773c 9896#ht_12684wt_1398
but I'll have to consider whether I want to take the risk on those.