Is freewheel a better a way to go than a coaster brake? What about the stem?With the brakes you already have, I would go with a freewheel. Use your existing wheels unless you want to be able to go back to the 6 speeds. Otherwise another set of wheels might be the way to go. Getting hubs, spokes and rim and lacing them up or having them done will be much more expensive than a set ready to go. With a 44 tooth chain wheel, a 20 tooth cog might work well.
Or you could just remove the derailleur and shorten the chain and use one gear on the cluster. We used to do that when our cheap 10 speed gears gave out.