I've always preferred fixed racks when possible (ie. can be serviced w/out gymnastics to get to it, and by those technicians whom might be described as portly.)

I'd say minimum depth behind the rack is 3 feet. That's not going to be pleasant to work in though, particularly the bottom half of the rack. Ideally you can sit on a chair/stool/5-gallon bucket and comfortably lean over to reach floor, bottom shelves, other.

I would definitely be concerned about casters on ANY slope. A rack w/ amps etc in it isn't something you want moving around on its own.

I've cut down cable ladders to fit as well. Sawzall and a bi-metal blade will get it done in a pinch. I once had to cut the backs off a pair of MA racks because they were spec'd based on height of the front of equipment space, but ceiling was sloped following the roof line, so you couldn't actually push them in to place. "Leave the walk/design to the sales guy" they said. The sales guy didn't cut down the racks for me...