My wife has a 1991 Klein Kirsten which she loves and says is very light and nimble:
https://www.flickr.com/photos/476463...57634958564465
They are not at all common though.
I have regularly seen Terry's on Ebay for very reasonable prices, and that would be my first choice if I was in your situation.
The idea of a MTB frame with suitable road geometry is I think a very good one, but here it would really help to have a good knowledge of older MTB's.