I think this is an excellent touring bike! I tour on a somewhat comparable mid-1970s Raleigh Grand Prix. I gave it a triple crankset off an old mountain bike, and replaced the original wheels with some nicer 700C wheels and 35mm tires -- neither dimension is a problem with the original caliper brakes (which stop me just fine). I've never owned a bike with bar-end shifters; I suspect I'd get attached to them quickly, but the downtube shifters work just fine if you don't know any better.

My suggestion is to modify nothing and go on a short, easy, low-mileage tour for a couple of days on a route without super-challenging hills. Observe what bugs you and make changes to address that. You've got a fine starting point -- the changes may be quite minor!