Ask. Short and long answer, Ask. There hasn't been any real technological progress in the Mailstation since the original E-messages, it's just that a lot of old E-Messages were unlocked in order to salvage sales when companies got out of selling them. The modern Mailstations can be re-mapped to an ISP of your choice with a little effort, (look at the 2 year old "Mailstation and your own ISP" thread on the main Mailstation page) though only for one user. But older E-Messages are getting harder and harder to find.
You can still get Mailstations for $10 on E-bay though... So getting one for each member of your family shouldn't be too taxing.
OK, a quick model rundown.
E-Message = Mailstation = MIVO 100 = Mailstation 100 = MIVO 150 = Mailstation 150
MIVO 200 = MIVO 250 = Mailstation 200 = Mailstation 250 = a basic Mailstation with Wireless Access.
MIVO 350 = Mailstation 350 = a basic Mailstation with Wireless and Rudimentary HTML and Image viewing.
There are a few other minor differences (the MIVO 100 / Mailstation 100 / Emessage originally had slightly narrower keys, but were easier to discern from eachother.) but they are only marginally different.
My data collection started years ago, and ended years ago, so YMMV. But really, unless I see an ARM processor or color TFT in one of these things, then nothing major has changed.