I have just completed a major update of the How To Megasquirt Your 2nd Gen RX-7 article to include MS3 and MS3-Pro. New schematics for those ECUs are available as well as settings and a base map. Additionally the MS2 information has been updated to reflect the current MS2 firmware. I've also updated various other parts of the writeup with more information, an installation video, and corrections.
Do you have an old MS2 you're selling, or know anyone who is selling one for decently cheap? I was thinking of turbocharging my car. Actually, I've been considering it for a couple years now.
If you don't have the time to do it right, when will you have the time to do it over?
All I have as far as spare MegaSquirts is a "cursed" MS1 that just won't run anything right for no obvious reason and has been relegated to spare parts duty.
I'd think that MS2s would be available on eBay cheaply but I can't recommend buying one from any old random person because there is no guarantee they didn't build it as their first soldering project.