1) The mirage is familiar territory if you were around when 8-bit home micros were the new thing. If you're younger than that, be comforted that hex is not all that difficult, and you only have to set start/end/loop points with it anyway. Most parameters are decimal or on/off.
2) Get the manuals or you've no chance. There was a beginner's guide and an advanced user's guide - get both. MASOS was the 'advanced' OS, but isn't essential - it replaces the crumby sketch-pad sequencer with extra sampling features like reverse, but your Mirage will work without it. You do need some kind of OS though, as you have to boot the thing from disk every time. There's also a parameter chart which is essential for easy use, as the two-digit display is the most cryptic I've ever seen on anything. However, it's not difficult to get your head around - parameter number, value. That's it.
3) Make sure you can find compatible disks. It uses 3.5" DD, not HD. If you can find some 720k floppies somewhere, get them - if not there are ways to adapt 1.4MB disks but I'm not sure they work very reliably. I haven't tried.
4) It's mono out, mono in, and there's no way around that. It has 128K of memory split into two 64K halves for the upper and lower sections of the keyboard (adjustable split). Each half has 8 samples which can be layered and assigned across the half. There's velocity sensitivity for the filters and sample switching/mixing, and those filters are CEM so they sound pretty good. It's an 8-voice machine. Max sample rate is something like 40kHz but best results are done around 30. It's very easy to get aliasing and run out of memory. Practise and read the manual's useful advice.
5) They're cheap. Buy one, try it, and sell it if you hate it. I won't be selling mine any time, there are some great sounds to be had from it despite its humble specs.