I checked, and 750 euros is equivalent to 917 dollars. I'd say a Mono Evolver Keyboard in excellent condition is definitely worth $1,000. Here in the US, Ebay has one MEK with the old endless encoders in excellent condition for $900, and Reverb has the same in mint condition for $999. So, the price you found is reasonable. But the upgrade will cost you another 200 euros or so. In my opinion, the upgrade is an important improvement, and the whole package is still worth it.
I've owned two MEKS. They are exceptional instruments, and having the control panel of the keyboard version, rather than the matrix panel of the desktop, will change you whole Evolver experience. And whether you use the pair of instruments layered or as a two-voice, you'll have a little masterpiece of a synthesizer. I'm admittedly biased in favor of any and all Evolvers, but I'd say you wouldn't regret buying the MEK...presuming it's in excellent condition and the upgrade could be done by a reliable technician.