I'm thinking of buying an HP EliteOne 870 G9 AIO desktop (512GB SSD and 16GB RAM -- which is plenty for me).

It will be a pure-linux machine (OpenSUSE Leap), so I just wanted to check whether anyone here has already installed Linux on this machine? Are there any obvious issues I need to be aware of?

Specs follow:

12th Generation Intel® Core™ i7 processor
27" diagonal QHD display
Intel® UHD Graphics 770
512 GB PCIe® NVMe™ SSD
16 GB DDR5-4800 MHz RAM (1 x 16 GB)

Kernel must be new enough to support 770 graphics, probably won't be a problem.
With dual channel motherboard 2x8 GB would be marginally better than 1x16 GB.

Bought a HP Elitebook G9 model earlier this year.
Everything looked good on paper when I pressed the buy it now button. Speakers caught me off guard. Headphones work but I have no working speakers under GNU/Linux at the moment (They work in Windows). I'm fine with that and understand that I'm using very modern hardware and that's why. I can wait because I am actually very pleased with the quality of this laptop. I definitely don't regret buying this machine.
My point is, although everything may look good on paper, there may be some curveballs you weren't expecting like in my case, the speakers.
Good luck.

IME, switching from dual channel to single channel RAM can result in memtest loss in speed up to in excess of 44%. Always install RAM sticks in matched pairs in dual channel motherboards if getting full performance potential out of your system matters to you.

Here's an Alder Lake on openSUSE, but it's pretty old already, and on Tumbleweed. I can't imagine any serious trouble with 15.4. I don't see anything in Bugzilla to suggest you should expect any trouble from 12th Gen Intel.

@Knightron

Headphones work then it means ALSA kernel driver is working, speakers not working is therefore some configuration issue. May require some serious googling to find out what can be done. Probably you need to pass some extra options to your kernel module.