Yeah those early 70's cheap organs are great for that grungy, cheap organ sound, love it. Most of my gear is rackmount but like yourself a lot of synth sounds are used with freeware vsti's I've found over the years or the stock stuff that comes with Cubase. My studio is pretty much dismantled at the moment for various reasons. So my current setup is very basic and quite freeing to be honest any external gear I tend to record live as audio for now.
I have a ton of circuit bent digital gear from a previous obsession I like to play around with, noticed your mattel, always wanted to open one of those up. I think my favourites from that period are my Alesis SR16 drum machine and an old Akai Sampler I did. I worked my way up to that Akai with it being AC but it was a simple job to solder up a patch bay for it, I'd never sell it though too dangerous however It makes the most fantastical, glitchy noises from its test tone alone.
Completely bowled over by your ownership of Rick Wakeman's old Mellotron though what a cool thing and a true piece of history!