I wouldn't worry too much about parts in a MS, just check that every function works, go throught every pot, key etc one by one and see if it's 100%, if it's not, suggest a more appropriate price to the seller, people will still try to rip you off with even a broken synth these days.
As far as the filter goes, I'm not sure what it's like with the MS-10 but with the MS-20, there were two revisions of the filter, and early version used the custom KORG35 chip that you mentioned, and the later chip uses a more easily replaceable design. Maybe the MS-10 only used the KORG35 version, I don't know. But any good tech should be able to reverse engineer the design should something go wrong with this version, also you can by replacements here -
http://www.synthrestore.co.uk/products.htm - Just scroll to the bottom of the page.
I think you should try not to pannick here, the MS range are quite possible to be fixed by practically any half-decent tech. Tuning is definately no reason to pass up a deal on a 10, all that means is that it needs a calibration as per the service manual, a fairly easy job and nothing to worry about.
Seriously though, just look at it this way, if something does go wrong, most likely it'll be an easy fix and not cost very much, until then, supposing everything works now, just enjoy it. Reliability issues go with the territory with vintage, if that's too much of an issue for you then get something new.
Cheers
