I have the A9. Some thoughts:
The A9 is a well-built Laser, sturdy frame, nice mechanics etc. Though, if you cut 20mm material, 40W is not enough in my opinion. Even with my 70W S70 I usually don’t do that. It’s possible, but it takes time and depending on the material, you have quite some burn marks on the back side. I‘d say it depends on the use case if 40W fit here.
Always use USB to control a laser, ignore all WiFi and SD features. They don’t offer full control and make connections less stable. So don’t care what features the laser offers next to USB.
Overall Sculpfun has premium user service, so it’s one of the best brands to choose. They care about any issue you could come across.
Here is some information about the A9: Sculpfun SF-A9 - Diode Laser Wiki
Adding the IR-2 laser head or the rotary / chuck is not an issue at all.