>>2838409The easiest way to fulfill this on a budget is a 90s rigid MTB. The frames are steel and strong. They will have rack points front and rear for racks (racks and dry bags are the cheapest set up by far), bottle bosses, etc. Flat bar is easy to add alternative grip positions to easy to mount bags to. 3x7 drive trains are dirt cheap and offer you both a granny gear for hauling up hills and a decent big ring so you aren't going incredibly slow on flat pavements. They have reasonable geometry for all day riding, and they fit typically up to a 50mm tire which is lots of cushion for gravel and such. V-brakes with good KoolStop pads are plenty for stopping even with weight.
For the commute it will still work great, racks make it easy to carry anything you need from spare clothes, to groceries, big locks. Ain't nothing that a classic 26in MTB can't do.