replace grindstone wheels or new bench grinder?

The message from "long ironer" contains these words:

Get the 200. Much easier to use.

There's an entire act of parliament dedicated to this topic (search for "abrasive wheel") It's a legal requirement that a poster-sized copy of the relevant bits of this act be displayed anywhere commercial where there's a grinding wheel in use.

These days, for a modern 6" wheel, you just slam the things on. But back in the olden days, when wheels were huge, egg-shaped, unbalanced and the holes were cockeyed, then there really was a lot of serious work to mounting and truing a wheel. And if you got it wrong it had a fair shot at trying to kill you.