The old Mac tower is fine, if you know it runs what you want to run. I have a 12 core one, and the issue I have with it is more that it bakes a room…technically it’s fine, but not exactly efficient computing. If you are setting out to buy a cheap system, I’d probably just get a m series Mac mini and call it a day.
I also used to run an HD24XR as converters, but I moved on from that as I didn’t have a need for that much I/O anymore. I used an m audio lightbridge (for sale if you want it..) to connect via adat. Again, technically this setup works (if you’re in the right OS, software versions, etc) but not real efficient or the best way to go about doing things. All this stuff is getting pretty old, in any case I wouldn’t recommend investing a lot in it or trying to run a business off it, but if you find this old stuff fun go ahead.