the mx50 was the top of the line. If you can get one for cheap, go for it.
but there are other mixers to consider too.
here is a good overview of some:
https://community.lzxindustries.net/t/what-are-you-using-outside-of-your-lzx-system-for-video-mixing/804/3
Personally I get very cool results from mx10 mixers (buy them for 5 to 50 euro). the effects are funky and the keying is cool. also; machine feedback!
The mx30 has some nice features too.
if you want something more modern, look for the Edirol options