Considering a refurbished Lenovo ThinkCentre M920q for use as a discrete HTPC / SteamLink console. Is this a good idea? Does anyone here have any experience with these?

I'd recommend against it personally. The user inputs with any reasonable os you choose will be finicky and that gpu is going to struggle with decoding the video stream while also pumping it to your display if it even supports the full resolution of your display. So, if you want my opinion, you're better off getting a cheap android TV box/stick that supports the full range of media protocols and will have a much simpler and better decoding support. It'll use less power and will save you a lot of headache in the long run. Nvidia Shield devices are the "gold standard" among av nerds, but generally anything from a reputable brand should be fine for your uses, just avoid going with the cheapest unknown brands from China as they have a reputation for having malware loaded from the factory. If you have the device just sitting around though, can't beat free I suppose and you can always change it up at a later date if you run into issues.