Daniel, I can go on and on on the subject...
But you need to make a decision.... so, I'd simply say: tell the guy: "I will buy one unit (the good one) for $300 ...(keep in mind $350 maximum). Period." If he sais: "no", then Tell him: "well, I'll have to think about it", and end the conversation. If you get contacted by him again ...then you may tell ...:"OK, $350... that's it. I can't pay more."
If he sais - NO, Then forget about it. Just let it go, really.
If you have $400 to spend or so... just hunt for 38 or even msr-8 .... you can get one... maybe not as soon as you wish...though
btw, I'd say there's nothing really wrong if someone asking somewhat high price for a vintage machine. There may be something more to it than just "market value".... you know how it goes. I can tell myself that I would not give up any of my vintage gear for anywhere near "market value" - forget it! no way. Well, unless someday I simply give up on all this crap and simply will have no interest what so ever. This happens to people all the time - boiling passions sometimes evaporate and disappear like morning fog in and you've got the "best seller you can find" ....
the bottom line - look for a seller who has no interest in the item he/she is selling, even better yet - look for a seller who has no interest nor knowledge
If a guy tells you a story about old days, values and about how great the item is/was etc etc.... you have very little chance on getting a good deal, and as for so called "great care for the unit" through out the years of use and love - I'd say: "yeah, right!", we all know what loving care may look like in real WORKING studio.... so.... well, you know
ohhhh, also, I may be wrong, but I think that presentation of "hard evidence" would not do much good for you in the situation.... Just name your price, deal with the situation on the "sell-buy" level... no deeper than that, if you know what I mean.
/respects