A major difficulty was finding compatible (and affordable) motors, gears, wheels and axles - it was rather harder then than it is now. I tried some cheap, tacky, self-assembly plastic gear boxes, none of which ran very smoothly, and eventually invested in a couple of reasonable gear-head motors that weren't too expensive, with 2mm output shafts.
I was lucky enough to find, all from different sources, inexpensive plastic gears that were a push fit onto 4mm axles, 4mm steel rod and 2mm to 4mm converters for the motors. In my local DIY store I found usable plastic wheels, and at a model engineering fair I picked up some dies for threading the 4mm rod (and acquired a new skill).
The brass converters were a push fit on the motor shafts, and needed super-gluing in place. The bore in the wheels was too large, but with spacers made from surgical tubing, cut overlength so they would compress as the nuts were tightened, a decent fit was obtained.
All this just to be able to put together a set of running gear. You can see the result in the photograph, along with some Meccano pieces used to support the axles.
I considered various available castors for the front but they were all too big or a bit clunky, so I ended up making a simple dolly wheel from bits of Meccano.