Hi everyone!
I'd like to build a bikepacking bike based on a vintage frame that I could use on a variety of surfaces - mostly roads, but I would like it to handle some light gravel too. I was wondering whether you would recommend choosing a vintage mountain bike frame as a base or to go for a vintage race bike with downtube shifters and try to fit thicker wheels. Furthermore, what kind of wheels/tyres would you recommend?
Given your goals, it sounds like you are more oriented to "bicycle touring" (road and some gravel) than "Bikepacking" (typically more rough / singletrack / backcountry type riding). Maybe I'm splitting hairs, but to me pavement with a little gravel is not bikepacking - it is touring.
With that in mind, any vintage touring bike should fit the bill. Some of the most beautiful and practical bikes ever built, IMO.