I find the 100mm length ideal for use with a butterfly bracket and a flash. I've had the faster Pentax SMC-A 100mm f/2.8 Macro for 20 years, the image is much brighter than the f/4 K version. Still it can be a challenge to focus with the helix racked out at full extension. Mine has the reputation of being better corrected due to the FREE focusing (which stands for Fixed Rear Element Extension, the rear element group moves near maximum extension). It goes to 1:1 without additional extension tubes and IIRC the f/4 PK Macro goes only to 1:2 (half life sized).
The reputation of the SMC-A f/2.8 lens being legendarily sharp is well deserved, I can't speak to the f/4 PK version. But I can tell you any K version won't work as well with all exposure modes using Pentax bodies more recent than about the LX of 1980, without the lens information contacts or the "A" setting.
You're rather unlikely to find one for the $100 price mentioned, though. The PK 100 f/4 Macros typically go for twice that, and the f/2.8's for $400-800 depending on condition. Either is rarer than the 50mm f/2.8, also wickedly sharp and contrasty, and which seem to sell for about $250. The 200 f/4' ED's are another matter entirely, there's one at auction right now that'll likely go for close to $2k (It was only about $700 when available new--they were very, very scarce even in 1990).