Well, judging by your knowledge I would not suggest that you try to hook up a MH retro kit. If you mess up the plumbing, the floor gets wet. You mess up the wiring, you get electrocuted, the house burns down and other bad stuff.
So unless you know an electrician that would be willing to hook up the system for you I would suggest that you just stick with the 'out of the box' systems. Even with retro kits you will still have close to $200 in each light. So there's really not that much difference.