I don't want to part with my original Sega Programmers Manual by Brian Brown, but you can download a pdf from here if you like. http://www.sc3000-multicart.com/downloa ... Manual.pdf
That is a lot easier to use than the SMS Power link you put up in the Classic Computer forum. It was also scanned / created by Aaron Wheeler. He did a fantastic job with the OCR, so you can even do text searches in it with a reasonable chance of success. I hope he doesn't mind me posting it here. But this is a great resource if you want to any programming on a SC-3000 so it is good to have it available.
As for the SF-7000, you just need to be patient. There are generally 1-2 of those on TradeMe each year, and they go from anywhere between $150 and $400 depending on how much stuff comes with them (usually around $200).