Another vote for the 2000/2100; the only Newtons that are actually good to use, even today. Lesser model Newtons are cool to look at and play around with for a while, but come up short in many ways (in particular software - there isn't much stuff for Newton OS 1.x). If you get a 2x00 model, make sure it has a serial dongle which are a rip-off if purchased separately.

The eMate is cool but clunky, although a nice budget option as there are plenty around for sale.