The main failing of older ones is lack of memory depth.
The main failing of cheap modern ones (USB streamers) is lack of width.
For looking at old machines, you want enough width to get data, address and control bits captured in state analyser mode. Some chips with multiplexed address and data bits can be used with narrower capture, provided the analyser allows you to acquire the same pins with different clocks.
I'd say the usable HP ones start with the 1630. They're big and nopisy but not as bad as the others of their day - Dolch etc. and they have enough width and depth to capture a useful instruction stream. 1650 / 16500 is a nice update but still a bit shallow depth for serial data - in fact, I'd use a cheap USB analyser for that. The 16500 A is floppy only. B adds hard disk (network optional). C adds network. Note that some of the plugin cards will only work in certain combinations - you need the later scope timebase to use with the later acq cards. Also note the analogue capture is only 6 bit. 16500 is a single knob and a touchscreen, the others have proper panels. As a result, it's much nicer to use with a mouse and keyboard.
1660 and onwards I'd think of as modern. Lots of other people here won't, and they're right

. I don't know much about the later ones.
All the above are CRT based. Not sure when LCD started. Also, at some point they started using Windows. Those tend to date faster imho.