It's a good CPU for retro computing, as it can be slowed down to the level of a 386DX using cache disabling and multiplier changes. The K6-III+ and K6-2+ can do this in software, so grab one of those CPUs instead if you can. I probably wouldn't bother spending much extra for the K6-III over the K6-2 either, but if it's a decent price then go for it.

Depends on what you want to run on it, and what games you want to play on it.

Windows 9x: A faster processor doesn't hurt. Depending on the games, it might even be too slow.

DOS: Could be too fast for some, mainly older, games. A slower K6 or K6-2 will likely have the same problem, though.

Buy a K6-3+ instead. The Plus version has software adjustable multiplier and L1 and L2 caches. You can bring it down to 386 level via command line.

I think I may skip at least the AMD K6-III processor. I have a couple of Pentium processors (133 and 166 MHz) that will probably work, so I think they may be better. Already have one newer computer with Celeron 400 MHz processor and the AMD K6-III would probably give similar performance which is quite fast already for older games.

I recently got a K6-III 400 CPU, in DOS it is a few FPS faster than a K6-2 450 MHz, but in real use I doubt you'd notice the difference at all. It is a neat chip, and I think it would do a lot better in windows than the k6-2, but not necessarily games, I suspect it's strong point lies in more serious applications and maybe browsers.