Hi Darragh,
You're in the same fix as many of us with "older" bikes. Until my Sherpa, my newest bike was the '89 Miyata 1000LT with index shifting and cassette freehub; all my others use friction shifters and freewheels (the tandem is has indexed shifters and a freewheel).
The trouble is, freewheels are no longer widely available in a range of sizes. I would suggest considering a freehub/cassette combo if you're going for a new set of wheels, but be mindful -- you may well need the shop or a framebuilder to respace the rear triangle across the drops and then reset the dropouts so they are parallel. A 9-speed hub will give you the option of running a 9-sp cassette or -- with a spacer -- an 8-sp. You'll likely need a new chain as well if yours isn't compatible.
Your bike is (in my opinion) a nice one (by memory, I'm guessing it might even be white in color?), and should have some good touring life left in it. Given you're shooting for a Sherpa to replace it, I think I'd set my sights on some good and reliable but basic components to keep costs down so your "hope chest" can grow a bit.
Along those lines, a good setup would be a pair of Alex Adventurer 700C rims, 36-14g DT spokes laced 3x, and some basic Deore hubs. I'm sure others will weigh in with equally good combinations and I'm happy to suggest others as well.
I built up a set of wheels like these for a friend in a similar situation, and he has found them basic but satisfactory and durable for touring on his 700C touring bike. They're not particularly light, but they are reasonably stout. He weighs close to 200 lbs/91kg/~14st, carries 55lb/25kg loads on everything from paved roads to gravel and some light summer dirt-trail use, and I haven't needed to retrue them yet. He's pleased, and it salvaged his older bike for a few more years. I did have to spread the rear and reset the dropouts to make sure they were parallel, but it was a minor cold-setting operation and it checked out as true and even to centerline on my framebuilding fixture. Rather than upgrade his shifters and derailleurs to index, he was content to leave the old friction-shifters and mechs in place, as he was already used to them and this kept costs down as well. Like you, he's looking for a "dream bike" of his own but it will be a couple more years before his custom-bike becomes possible. Meantime, he can still have fun.
If your front wheel is still basically sound, you may be able to get by with just a new rear wheel if you really wish to minimize costs. The money saved could fund a lot of enjoyable tours in the meantime.
Darragh, there's a lot you can do cheaply to keep your present bike going, and -- provided it is otherwise sound -- keep you on the road and touring safely and happily for some time to come.
Best,
Dan.