Get a Japanese Sega Master System, its cheaper than tracking a SC-3000 down and works just fine
So you can play SC-3000 games on JAP Master system? cool. 
Yep, The SC-3000 is based upon the Mark II hardware, which is an extension of the SG-1000 hardware, they share many of the same hardware, just one has more ram, and they are backwards compatible with each other all the way up to the Japanese Sega Master System which is an Enhanced version of the SG-1000 Mark III.
Actually the "Mark II" is exactly the same as the SG-1000. Sega never released a console called the "Mark II", you're thinking of the SG-1000 II. The only real differences between the original SG-1000 and the SG-1000 II are the cases and the SG-1000 II has removable controllers for both players, whereas the original SG-1000 has a hardwired joystick for the first player. The Mark III was never called the "SG-1000 Mark III" either.
Anyway, I'd second the request for SG-1000 and SC-3000 covers. I've got a few SG-1000 games so far and I'd like to case them up!
I know it wasn't called the SG-1000 Mark III, I was using that to denote its part of the same series. But the SG-1000 series (1000-Master System) is like the Atari 8-bits, get the most recent model that is Backwards Compatible and you'll be fine with playing the previous games.
And before you ask, yes there is a Japanese variant to the Power Based converter, and No, it lacks the ability to play SG-1000 games, only games made specifically for Master System (Mark III is unknown) because the Genesis/Mega Drive cant run in video modes 0, 1, 2, or 3 which are required by the SG-1000, and Mark II.
