Wow – what a great gift idea. Lucky guy!
I love the R107, but the market has gotten a bit heated in the last few years. Nowadays, it takes mid-teens generally to find a nice one, and super nice examples go for mid-twenties and more. You might be able to find an older example (1970s 450SL) for under ten grand, but I think it would still more likely be somewhat needy.
What would she think about something from the 1990s instead? Most models there are still pretty much in the depreciation doldrums, so they’re as affordable as any classic Benz out there. For under ten grand, she could get a really nice version of an R129 SL (though those are also getting harder to find at this price point). Another thought is the W124 coupe (e.g. 1993 Mercedes-Benz 300CE Sportline - 104k miles - $6500 - Dallas, TX)
If she really wants to stick with the 1980s/70s, I’d suggest looking at W123 coupes and sedans. Lots of them are diesels, which might be a turn off. But gasoline variants pop up occasionally – and thankfully are usually cheaper than the oil burners. Especially cool are the Euro versions (with slim bumpers, flush headlights, plaid cloth interiors, etc.). If they don’t have super stringent smog regulations where they live, one of those could be a really good option. For reference, here’s one that I had my eye on, but the circumstances weren’t right: http://www.mercedesmotoring.com/cars/225/
I think it ended up selling for about $9000.