The E-Synth keyboard is being phased out. It is also not upgradable to an Ultra series. The Ultra models are very similar to the older E-Synth, E-IV, etc. models excep they have a faster processor and are more upgradable.
My recommendation to you is to buy something like an E5000 Ultra, which has the same synth/sampling engine as the E-Synth. The E5000 doesn't come with the E-Synth ROM board (ie online sounds) but you can add the E-Synth ROM board (if you like the sounds) as an option. You may also look at buying some sample CD Roms.
Now, I know that I suggested a rack mount module rather than a keyboard but you can use the N5 as a controller for it anyway. Unless you need to physical keyboards, you can access the E5000's sounds from the N5. All you would need is a MIDI cable to connect the 2.
The Ultras will be upgraded into the future but you'll be stuck with the E-Synth which, although it is a great keyboard, cannot be upgraded to the same extent. The Ultras have the capability of Beat-Munging (which is excellent) as well as the capability of adding the RFX-32 chip when it is available in the summer. This RFX-32 chip will provide a very powerful FX processor to the Ultra samplers. I would suggest reading about it at the Emu site.