Not a bad idea, but not exactly the direction I want to go. I'd much rather build this machine and modify it to meet my needs in the easiest of methods without a bunch of engineering and fabricating.I have a Cub Cadet Enforcer zero turn mower with a set of chains for the drive wheels. I paid $2700 for it with 254 hours on it. With the chains on, it's just about unstoppable; there's only one spot on my property I won't take it into- the area adjacent to the shallow end of the stock tank. It's black mud a yard deep.
Accessories like a blade, scraper or such wouldn't be hard to adapt or build.
If you really want a tracked vehicle, it might be easier and cheaper to start with a zero turn (which has hydro drive and tank controls), fab up a front idler axle, drive sprockets to replace the wheels and run a set of existing tracks from a bobcat or even from one of these. You'd end up with a platform at a lower cost, and you'd have base where you could buy parts from the manufacturer.
I actually have been looking online for a used one for sale, and they do pop up occasionally, but so far no luck there.It's not a replacement for a good full sized tractor, but for small stuff around the house, it looks like it should be fine. Before you pull the trigger on a $5K purchase, I'd look at a few used equipment websites just in case. You might find exactly what you're wanting for a bit less.