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>a project like this
Like what? All you've said is that you want something not bottom tier (fair)
What kind of riding do you want to do? What ideas about a build do you have?
If you want to run 700c wheels thats fine, buy a 700c wheel bike. Or a 27" bike. Look for a sport tourer, or a butted cromo hybrid. I don't think you even want a rigid 90s mtb if you hate 26" wheels.
A lot of old road bikes have a lot of clearance. I just got pic rel and because it's designed for fenders, and larger 27" wheels, on 700c it fits 38s. So it will make a good gravel bike.
It's kinda hard to give you advice about how to find a good project but maybe if you don't think one exists then you're looking in the wrong places.