I'm fairly sure the tractor is a Satoh S370, manufactured in Japan. They are good little tractors, but are little. I can't help you on price, but $5500 seems pretty high.
As far as parts go, it depends on what you mean by "available." Clutches, water pumps, engine rebuild parts, belts, filters, tires, etc. are all available, but will almost certainly require an internet or phone sale to obtain. The hard parts won't be found on the shelf at a local dealership unless you already have done some groundwork to obtain part numbers. Filters and belts will be available at Napa, and tires are tires. If you're wondering about things like replacement large castings, they may be available sometimes, from tractor recyclers, but nobody can order you a new casting from Satoh. Some sources may be able to find somebody in Japan who has some old stock somewhere, but this is a 30-35 year old machine. There's not a lot of call for big replacement items anymore.
For what it's worth, Satoh made excellent little machines, and the Mitsubishi diesels in them are great too. I wouldn't pay what he is asking though. Look around in the neighboring classified ads and find some other options and get a better feel for the pricepoint of similar machines before making a decision.
