...I still have a couple of bikes with Helicomatic hubs on them, but you are correct in not wanting to get stuck somewhere on a touring bike with no choice but a new wheel.

I've swapped out a couple, and the bike setups I've had exhibited no problems, other than requiring some derailleur adjustment and tweaking. You can rebuild your rear wheel, but even if the rim is in good shape, it's likely you will have to buy new, different length spokes at considerable expense. I have always just found a set of used hubs with the OLD's of the proper width, bought new rims and spokes, laced up the new set, and kept the old ones over in a corner in the pile of wheels that are too good to toss, but probably won't get used.

I'm pretty sure your 6 rear cogs equate with a 126mm rear. Most of what I find used here in that dimension uses a freewheel, not a freehub. But I know you can buy freehub hubs in that dimension, and it would not surprise me if someone is selling replacement wheel sets prebuilt. It's possible to reset the dropout distance on your frame larger, to accommodate more cogs, give you a few more usable gears, and still get through all of them with your DT shifters if they are friction, but I generally do not do this because my interest in such a bike is overall range from high to low, which I can do quite nicely with 6 on the back and three in the front. In selecting your new cogset, it will probably shift a little better if you get something a little newer, with ramped teeth.