Depending on the hub you select, you will have to 1. accommodate the cable to the hub, which may be either to the right chainstay or the left, depending on the hub you choose and, 2., you will have to find a way to properly tension the chain once the hub is installed. Tensioning can be done by a jockey wheel spring tensioner, as with a derailleur, or by a horizontal dropout (which may mean having new dropouts brazed in--the Venture GT is Reynolds 725 steel, I believe) or by an eccentric bottom bracket. The jockey wheel would be the cheapest but "dirtiest" solution.
Costs: hub of choice, with cables, and shifter, barend or other mount for a shifter (probably would have to be mounted to your stem), costs of wheel rebuild, costs of jockey wheel tenisoner, costs of new chain.
Alternatively, delete the jockey wheel, add the brazing and costs of bottom bracket replacement and respray work or add the costs of an eccentric bottom bracket. The eccentric BB would be second cheapest.