I reckon a little lathe should be purchased(cheap chinese bench thing) for making axles to fit small sealed bearings and I am thinking home brew wheels that are flanged front and back for stability. I would definitely separate the driving wheels from the pick ups and tyre bands for grip.
Like alot of things; resistance is a problem. You have mechanical, wind, heat and electrical in this case. The 'test mile' and lead up track needs to be as straight and flat as is possible. The track needs to have as little variance of voltage along its whole length and of the best material for conductivity. Same goes for anything involved on the electrical side of things regarding resistance and conductivity. Weight isnt a prob if you can have a run up so maybe maybe make a bespoke flat floor(low CofG) out of alloy to use as a heat sink for the motor(s) which may see little gain in performance over such short runs but should extend the life of the motor(s). The flat alloy floor is good for aero, longevity, and stability all in one go.
Aero is cheap to experiment with. Flat bottom the underneath as above, fair in the wheels and flush glaze or tape windows etc. Every surface jut needs to be smooth step/indent free and no sharp changes from one surface to another. Hatchbacks please leave the room!
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As with alot of performance things(machine or human) there is alot to be had in marginal gains, once they are all added up, before lashing out on expensive shiny bits.
Inglenookfan