I would consider an older steel hybrid frame, eg. a Bridgestone, Specialized, Trek, Schwinn. I have a Trek 720 Multitrack errand bike that's a great rider. I have it set up with flat bars, 1 x 6 drivetrain (I'm in the flatlands), racks and a front basket. The possibilities are endless with these bikes and they come up frequently on CL for not a lot of money. Put a decent wheelset on them and they will surprise you.