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If it were me, I'd favor an aluminum rigid with 26 inch wheels, rack & fender eyelets. But steel is just as well.

Then I'd junk whatever knobby hard rolling mountain bike tire was on it in favor of some 26x 1.1 slicks up to 26x1.5 commuter tires.

I favor 559-28 Continental Gatorskins (aka 26x1.1 or 26x1 1/8)

For in city use anything bigger that 1.5 or has knobs really isn't necessary, gets heavy, and rolls slow unless they are super light & thin; which disqualifies them from "commuter" purposes, IMO.
I'm sure many will have their own opinion.

I'd avoid vintage anything with suspension. That's just like asking for old, obsolete, worn, broken garbage. I'm not saying there might not be a true gem of garage queen wonderfulness out there. I am saying that the potential for problems & expense just isn't worth the risk or the trouble.

Good luck.

Oh, FWIW: check out Bike Works in Columbia City, on Ferdinand Street. What you are looking for is their specialty.