Turntable for Mom

Either would be good. There are also "entry-level high-end" brands (Rega, Pro-Ject, Music Hall) with tables in the $400 range, though I don't think they're substantially better. They all come with the cartridge attached, so set-up typically involves no more than putting the belt on and setting the tracking weight. Both are easy.

A key difference is that the Denon is automatic, which means you just press a button and the whole thing starts, and it shuts off when the record's finished.

For a manual turntable, the start-up process requires several steps, including moving the tonearm into position over the outer edge of the record. Depending on your mother's age (eyesight, hand steadiness), that might be a problem. Having to jump up as soon as the record ends might also not appeal.

That I can't tell you. But isolation is going to be a problem with any turntable in this price class, and being plastic doesn't automatically make it any worse. Also, if automatic operation is important, I think this is the best you'll find.