Look this machine over very carefully. I see alarm bells in your description from the first sentence on.
First of all, 4,000 hours of use in ten years is absurd for that size of machine. That kind of use is usually done in three years. I've found very few people that would put out that kind of money to run a machine for 400 hours a year. I have no doubt it has many more. Check the condition of the muffler. If it has been changed the machine likely has over 7,000 hours. Many people cut the stack off the muffler and clamp or weld another one on. Look around the engine bell housing. If it is black from exhaust soot you have an indication of a rat. If it has new paint, make sure and run the machine at high idle and look for exhaust leaks.
As far as the price on a thumb goes, you need to state whether or not that is installed. Your machine is probably using the extra valve to run the blade. In order to use the thumb you have to install a diverter valve to run the other circuit. The thumb will have to be fit to your existing bucket and you will have to run new tubing over the boom and install the cylinder to the stick. If you can get all that done for $8,000 or less you should jump on that deal quick.
The Kobelco dealer in Portland is Feenaughty Equipment. They aren't bad to deal with. You might be able to get the work done through Jewell Manufacturing. They build up many of the logging machines that Feenaughty sells. Weldco Beales, PSM and Highline PortaFab are three other manufacturers of thumbs. Weldco Beales is in Tacoma, WA and both PSM and HPF are located outside of Woodenville, WA. In addition there are a few startups doing thumbs and attachments all over the Northwest. You might find a deal by doing a search on the web.
Kobelco excavators are not bad machines, but there are a lot of bad operators and owners. Check this one out very carefully before laying down hard cash.