For all the work you intend to do to a second-hand MTB to make it as close to a roadie as possible, why not just get a second-hand roadie?
What you wanted to do with tires reminds of a quote from an illustrious forum member:
Putting slicks on a mountain bike is like putting a dress on your best friend Bob. He still won’t look like a woman, and he won’t ride like one either.—
Platypius
For the past, I dunno, forever it seems, entry-level roadies have all included rack and fender mounts. Four years ago I picked up a second-hand Trek 1000. There's enough room in the frame and fork for 28mm without fenders, 25mm with. I run 25s.
All I had to do was install the rack, fenders I put on and off according to season, and I replaced the brake pads with Kool-Stop (salmon) for the wet. Along with lights, which I swap among all my bikes, and it was all set.
The other nice thing about it being entry-level, is that it's 8-speed. Chains last 1,000 miles longer than on my 10-speed, and cost about a third.
I did eventually upgrade the wheels, but it was because I wanted to, rather than needed to. I stayed with 32-spoke in the rear and they're still true as the day I bought them, three years ago.
