Re: [DIY-CNC] need transmission ideas / advice re: gantry

Couple more cents to add:
Starter motors, as has been pointed out, are very low duty cycle, 10% at best.
Electric window motors are also quite low, something like 50%.
Wiper motors are at their very limit at 100%.
Neither of the above 2 put out anything like enough power, a pair of large
truck wiper motors would be close though.
The hydraulic power pack motors come in a wide range of sizes, up to about 2hp
at 100% duty (not cheap) and the supplier has the power/duty graphs.

The circuitry for 12 or 24 volt is not going to be cheap, it may work out less
expensive to buy a complete treadmill and have everything you need (motor,
power supply, regulator circuit etc.) as just the regulator circuit for the
current you need at 12v may cost more than a complete treadmill. If you do go
down the 12 or 24V route it may be worth considering using batteries and
chargers as the power supply, crude and bulky but a fraction of the price of a
high output inverter.

Good luck with it.