RE: How much for an original vintage Tamiya Thundershot?
Disassemble, reassemble, polish every part (not literally, lol), make sure its in perfect working order, make a note of which parts need replacing, be up front about it with the guy, then hit him for several hundred euros. Start at 300, don't take less than 250. There's a new in box one on ebay for $650.
It's worth whatever you're willing to sell it for and he's willing to pay for it, but its a rare original, so don't let it go cheap.