Hi Nalu,
First, welcome to the boards, from NJ!
According to the salesman who sold me mine (on an Oceanic Versa Pro console), the Q D is good for:
1.) taking just the console with you to record info to your log book.
2.) taking just the console with you if you want to download info to your "regular" (non dive) computer.
3.) taking just the console with you to keep it "safe".
4.) Simplifying packing.
I haven't applied "2" yet, but the other three reasons seem to be valid.
While I don't feel that there is an absolute need for the QD ( Due to circumstances, I didn't have much of a choice), it isn't a bad thing to have-when it performs as advertised. However, it is another potential "failure point"on your rig.
BTW, unlike "the old fashioned way", most of the HP hose is left attached to the first stage.
Regarding a good website, there are far more experienced "on line shoppers" than me on this board, and I'll defer to them. Usually, where they sell the computer,they sell the console/compass/stc.
{Oceanic has it own website, but as to prices and service, I'm not sure.}
Good luck with your DMPP,
Mike
ps: I see that ,once again my snail-like typing has resulted in other answers getting posted while I "pecked away"
pps: I agree with wch's reply.