I can't help with much of the information that you are looking for but I am sure google would be happy to help fill in what I can't.
What I can say is that I own a Suzuki Grasstracker (more or less the same frame and engine as the TU) and I have found it to be a great bike around these parts. There is no registered sale of these bikes in Thailand as far as I am aware which means they are almost always grey market imports without proper plates. Personally I have not found this to be much of a problem but you can read up about plateless bikes in other places on this forum. Getting parts has not been a problem though most of the parts that I have used have been from other bikes. So far 30,000km later, I have not needed anything major but I know that if I do, the bike is still made and sold both in the US and Japan so ordering parts is an option (though not as cheap as locally made parts)
I would suggest looking around some of the Thai sites most notably http://mocyc.com for used bikes. Search for TU 250, volty, and grasstracker and you might come up with something you like.
The prices seem to range from 30,000 - 50,000 depending on the condition of the bike and the amount of custom work that has gone into it. That is for a bike with out a place. I have never seen one here with a plate but have been told that it runs about 55,000 to get a plate for them. Makes it a hard thing to do when you can buy a brand new 250 for just a little bit more money.
Hope that helps and if you have any other questions, just let me know. Happy to help.