I like the thought process of mixing in a real mixer, but your suggestion would work. I don't feel you really are needing the HD recorder, but might be better to get a stable PT or Cubase or Nuendo setup instead if you are mixing ITB. It can be done.
When doing overdubs, you need to mix in a PC if you don't have a mixer so you need all the proper I/O's to connect 24 tracks into your PC, so you need to have a lot of firepower available on the PC end anyways. And if the PC crashes during an overdub you're not much better off than someone using a PC/DAW setup. Alternatively you could use a smallish, cheap, 24-track mixer to do monitor mix on, and use the PC for editing and mixing down only, but that seems a bit strange to my way of thinking. And you'll need a way of patching things into the A-D system - preamps need to get repatched to the input of your HD24 manually so you'll need all that cabling and a patchbay from preamps to analog inputs. Using a digital mixer or a PC/DAW system you don't need to do all of that.