don't worry about it, if it's 125mm then cramming in a 130mm wheel is no big deal, the drop outs will spring apart that much. For 135m you should have someone cold set the chainstays if you're shy to do it.

It's a good bike. I had one for a short stretch and sold a few when I had a shop in the 80's. I'd have to A/B it with the LHT but i would think it's in the same league.

Go ahead and replace the dereailleur. Any of Shimanos touring derailleurs should do it. That Suntour/cyclone derailluer is nice but they tried to expand the cage while sticking with the smaller cyclone model parrallelogram and have that long arm/spring take up all the slack and it's too much for it.