I'd leave that bike stock and ride it for now. See if it really meets your expectations. Some early long wheelbase ATB's really weren't great rides for loaded touring or much of anything. Commute with it and if you need to dial it in with new componentry, keep all the original bits.
Some mild acid cleaning and waxing will keep rust in check. I think widening the spacing is a rabbit hole. There's lots of these wheels about, for awhile. Ultra 6 freewheels will fit.
And everything rhm already said