DataMaster uses Serial ports, while Diacom uses Parallel (printer) ports.
I have used DataMaster on a P100, P133, P166, and P233MMX and I would have to say that I would not recommend anything less than 133. A 100 or less might do the trick but it will be at a low sample rate where you might miss some frames of important data.
Once you get to 200 or better you are going to get the full amount of data or close to it, so that is the sweet spot. RAM is recommended at 32MB by the TTS company, I beleive. Hard disk space is not really important, I use 2GB drives and they work just fine.