For those who are going after MPX1s, definitely make sure it is equipped with the latest V1.10 firmware, or else you'll be dealing with a bunch of strange MIDI issues. Also make sure that the battery has either been replaced professionally or hasn't damaged the board from leaking acid everywhere destroying traces. Expect to replace this - preferably with a coin cell holder for future ease of maint. Verify the LCD backlight is bright and hasn't died, too.
Expect to pair a mixer with this unit, and be prepared to read the manual. The PCM81 is a much better manual to understand how Lexicons work in general, so if you're still confused after reading the MPX1's manual, try looking at that. It's actually a great manual in helping understand algorithm design in general.
https://freeverb3vst.osdn.jp/doc/Lexicon/pcm81_rev2.pdf
Remember it's a two chip unit. One chip is dedicated to reverbs while the other can handle additional FX. Very slick unit overall. Kinda sucks in mono, though, so you might be disappointed in a mono rig. Italo's presets are pretty awesome for the unit:
http://www.italodeangelis.com/it/eventide_and_other_stuff/lexicon-MPX-1.asp