I'm fairly sure MFOS don't have any polyphonic options. I don't know of any DIY poly synths by any manufacturer. Let me know if there are... Most of the synth DIY market is for individual effects, filters or modular synth modules. DIY kits for completekits are generally mono. Or able to do multi voices but you will have to use your cv or midi controller/sequencer to get poly voicing.
I'm building/ have built a few MFOS devices for a DIY modular I'm making.
Finished there 8 stage phase shifter (amazing effect), noise cornucopia (noise generator to be used in a modular system)
still building/troubleshooting the MFOS Soundlab which is basically a self controlling synth for wierd noises and drones
I've made my own front panels.
The paia tabletop mono synth (can't remember the name) is highly rated and comes with a complete rack or tabletop chassis. Its midi aswell I think
thereare some good complete synth style DIY projects at the elby designs website too
Also to answer your questions about MFOS components, the guy who runs the MFOS site designs his circuits around affordable and regularly available components on purpose to make it more affordable and easier to put together for the average DIY-er.
If your after better component performance then just by the pcb by itself and source your own components. So you could upgrade the TL072 and TL071 etc chips with better op amps, buy metal film 1% resistors rather than carbon film 10% resistors and source more accurate pots. Use better film or silver mica caps for the non-polar caps etc.
Perhaps the best TB-303 clone is available as a kit. The x0xb0x. I've built a couple, and they are superb thumbsup
Originally produced in open source flavour by Adafruit. x0xb0x: Transistorize the World
Except they seem to have lost interest in it. But no matter, as kits and parts are available from several other places now.
Like here: Willzyx Music - Welcome or here: über x0xSh0p.de to name but 2..
The Shruthti synth kits look great too. Im hoping to get me one, but they are a bit tricky to get hold off currently.
The WTPA 8 bit sampler kits are very cool too. Theres a new version out, any month now..
Also, not a kit, but Korg monotrons lend themselves very well to modifying. I just got mine yesterday, so im looking forward to seeing what I can do with it