Anyway, I've dealt with vintage fun world in the past (bought a couple atari joysticks and a copy of Amiga World #1). No problems there. As for the price, it's not bad, although I tend to prefer the model 1 unit myself. As for it not being complete, it's everything essential.
That site is preaty good. I never ordered anything from there but I saw a Video Game shop review site and it has preaty good reviews; and about the mixing cable, its just a audio male to audio male, I think. You can get those at wal-mat for preaty cheep. As for the Extending plate, its not need. The MD 1 will look funny. I have a MCD 2 and its ok.
The mixing cable is only needed to get stereo sound on a model 1 genesis / MD (the ones with a full-size DIN plug for audio/video instead of mini-DIN), and is kind of a gross hack anyway. The plate makes the combination a little more "solid" physically but isn't necessary.