The only reason to go with an older TDM rig would be to work in surround. Any current native system will handle larger mixes than an older HD3 rig(and for that matter, if you found a 12.7.1 license for HD, the 003 could handle surround since it has enough outputs)
Maybe the more important questions would be about the computer you plan on using, and why 12.7.1 instead 2019.5? I ran a 6-core Windows(7) machine with 32GB or RAM with a DIGI003 and had no issues tracking full bands(up to 18 live inputs) at a 64 buffer setting. I mention 2019.5 because it(on my rig, anyway) runs leaner than any previous version. I'm doing full mixes now(even with Ozone and Melodyne) at the 64 buffer and I never could do that with any previous version unless I upped the buffer(opening Ozone or Melodyne would stop playback and throw an error unless I went to 1024)
