Retrofitting disc brakes on 2000 Specialized Stumpjumper FSR XC?
The caliper is touching the wheel because the caliper was designed for a disk to be there, but the wheel was designed for spokes to be there. With a disk-brake wheel, the spoke flange on the hub is moved closer to the centerline to make room for the disk, which would be where the spokes are now. There are only a few standards for mounting disk brakes, which determine where the disk and caliper should be relative to specific points on the bike. Unless you want to mess around with adapters, you need to make sure all your parts fit the same standard. I'm guessing your fork uses the 74-mm post mount standard.
This distance is called the over-locknut distance. Almost all bikes use a 100-mm standard for the front wheel. There's a more recent 110-mm standard (called "boost"). The fact that you're getting 103 mm is probably a measurement error, but that's a 100-mm hub, and you should continue to use 100-mm hubs.
Not really. It's not clear exactly what kind of rim is on that motorized wheel, but you can probably mount a fatter tire on it if you want.