It's not too difficult, just a bit fiddly sometimes to get everything out and in again. Not much more difficult than, say, a water pump or a timing belt.

A new rack is obviously new. A rebuilt is a used rack that's been taken apart, fixed, and put together again with new seals and all. Quality can range from almost as good as new to dreadful, depending on who did the rebuilding. Some companies consider just a seal job to be a rebuild, while others will polish the bores, replace the rams and gears, etc...

Bram