You will need to define the target budget, as your drivetrain alone could (rear hub/wheel setup and chain alignment) could be places you spend money after finding a good starting point from a geared bike find perspective. To minimize cost and/or complication, make sure you look for horizontal dropouts on the rear wheel. Otherwise you may be finagling with eccentric hubs and/or chain tensioners. That is not a bad thing, but just more $$ or maintenance, respectively. Getting an eccentric hub wheel build is the $$ part, however, i think it would be well spent if a vertical dropout is all you can find at a reasonable cost for a starting point.
I ride a single speed fully loaded touring rig (front/rear panniers). Touring geometry with a 42x16 ratio on 700cc rims. Can ride that thing all day long and super comfy.