Noel,
to use the tooth belt wheels as suport for the loco wheels is a good idea. There is more adhesion to prevent the wheels from slipping. Additional it is easy to connect the brake alternator. As brake alternator I would use DC motors from a car scrapyard and there i would collect the motors of the car radiator. They have normally at 12 Volt 30 Amp current consumption. With an estimated efficiency of 70 % you have about 250 Watt brake power of one motor and it is clear you need more than one motor. A second good source is a motor of an electric steering they are in a very good quality. The best source is a defect electric steering column. There are DC and brushless motors available. And they are in a range of 12 V 60 A with an efficiency of more than 80%
If you measure with a dyno drawbar a small wheel slip is not a problem.
Werner