Garbage disposal replacement: how much to replace?
I wouldn't rely on the old flange. While the flange itself might be fine, there's plumber's putty between it and the sink. That stuff doesn't last forever, and if you knock it loose replacing the disposal, you'll wind up pulling it off and adding it anyways. The good thing is it's not crazy hard to get off or put the new one on.
The dishwasher compression fitting depends a great deal on what they used and what condition it's in. I was able to keep the same one (rubber) when I replaced mine a few years back, but be sure to inspect it for cracks and leaks. It can't hurt to replace it either.