Take a look at the Classic & Vintage forum for some ideas. You don't say how tall a frame you ride now. For a frame larger than 55cm , 700c wheels work fine if there is enough fender-toe clip clearance. This may or not be true for a 52 cm frame(my size). For smaller sizes smaller wheels might be a better choice.
There is another possibility and that is 650B, a size intermediate between 26" and 700c. Some old bikes can be converted from 700 to 650, with long reach brakes. I would tour on 650B, but there might be problems finding replacement parts in case of wheel or tire problems.
If I were looking for a touring bike I would look for an old Specialized Expedition, a Miyata 1000, a Kogo Miyata, or a Miyata built Univega tourer. But there were lots of other Treks, Fujis, Bridgestones , Motobecanes, etc. that are probably just as good, and possibly better than most new bikes.