Nobody has really addressed parts avalibility or common problems this tractor might have that were possibly common for that size and model.
As someone mentioned, it is a "grey market" model... Kubota never sold it in the USA. You can check because it probably has Japanese on the serial number tag, etc. Also, it doesn't have the seatbelts, ROPS, etc. required here. It was imported privately by someone selling used foreign tractors. Officially, Kubota doesn't support those tractors in any way... here's what their website says:
"Neither Kubota Tractor Corporation nor its affiliated company Kubota Corporation of Osaka, Japan, provides parts, service or any warranty support for Kubota "gray market" units in the United States. There is no responsibility whatsoever either by Kubota or its authorized dealers for these "gray market" units."
If you go to a Kubota dealer and ask for a part for that model tractor, they probably will not have the parts reference.
That's the bad news, and another reason why it's not worth very much. For comparison, a legitimate 30+ year old US model Kubota of about the same size and HP, the B6000, could go for up to $ 5,000.
The good news would be that it was once a good, reliable little tractor (who knows what it's like today) and there are probably a lot of common parts shared between models, and if you didn't mind spending some time chasing around for parts, you might find most of what you'd need. So it would pretty much depend on what you want to do. If you wanted something to tinker around with and could get along if you had to let it sit until you find a part, it could be a cool project. But if you plan to use it all the time for work that must be done on time, well you get what you pay for.
By the way, the tiller is probably worth the $300 all by itself.