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Any free mowchine is a good platform to start with! Heck, my best one is a rear engine rider (who woulda thought?)!
The minimum safety requirements shouldn't cost too much. You can pick up everything used/cheap off eBay. Alot of the go-cart guys use the same things we do (brakes, spindles, tethers, etc). I've never had to reinforce the frames on my mowchines, but I could see the need to on some cheaper ones.
I think the weight of a garden tractor would seriously put it at a disadvantage, but at the same time if you can throw a driven front axle under it - you'd have an awesome recovery rig