I'm wanting to get the Thordsens Customs enhanced pistol cheek rest. My question is do I need to buy an optional buffer tube or any of the other options they list? I have a CMMG MK9 PDW in 300BLK. I've tried the search function and google and I found lots of install videos but not finding if my factory tube will work. Thank you in advance.

It depends. It comes with their keyed buffer tube. The optional tubes are extended variants and likely out of stock. You will need to purchase a carbine buffer, spring and end plate unless you have them lying around your house.

ETA: From what I have come to understand you can use a standard carbine buffer tube but you would need the buffer tube cover that the rest mounts on. Pistol tubes will not work with the kit.

I had the same question and this is the response I got from Thordsen Customs:

"If you would like a pistol cheek rest kit then with the buffer tube you
have you can order the CAA Pistol cheek rest kit and it will work with
your existing tube. The kit number is either 5022B or 5023B depending on
if you want the enhanced or the standard buffer tube cover.

Shari"