No, Grinding calipers is not reasonable and is dumb.
With so many tires available in larger wheel diameter, why 15s.
You can hardly find a used 15 now, they are really on the wane in popularity.
I have been looking a used set for my daughter's truck for weeks.
No, they wont fit. Period. Unless of course you buy wheels that have one heck of an offset so that the wheel mounting surface is flush with the inner bead... which they probably don't make and you hubs will not like...
Measure how much clearance is between your caliper and wheels you have now. If they are 17s you have less than an inch and to go to 15s will require grinding your brake calipers down. I have seen it done, but as a 20 year ASE Master Tech this makes me cringe. Brakes operate at very high pressure, I would not grind on a cast part in my brake system.
I feel for ya, I find deals sometimes too that just won't work with my setup. Welcome to NC4x4 by the way. Glad to have ya.