The C128D is a C128 with a built in 1571 disk drive. The keyboard is separately, which gives the C128D the look and feel of a real "modern" computer.
I don't know many technical details, but I hope to get some informations soon. As I discovered myself, it has an internal power supply, but not a conventional one with big and heavy transformer. Instead it has a switching power supply, like the ones in modern PCs. For this it also needs a fan to cool the power supply. There also exists another variant of the C128D, this one has a metal case instead of the plastic case, and it has no fan. The pictures here are all from the normal one.
The connectors are almost identical to the normal C128, except that we have the keyboard connector left of the game ports, and there is an additional reset button for the disk drive. The Power switch is on the left side of the machine and the power connector is seen on the picture most right. The two hooks are for rolling up the power cable I think. :-)
As seen on this picture the keyboard can be attached on the bottom of the machine, and there is a handle to carry the machine. But I would not risk that with the weight and the current age of the machine...