Some require the GMAT others do not. You can apply to all of them after your first year or two of medical school so if you haven't taken the GMAT yet, don't worry. As for work history, after you are in the medical school most accept your first 2 years of Medschool as supplement. I have spent 2 years at a bulge bracket investment bank so employment history is not an issue for me, but if you currently have no work experience, I think it will be overlooked if you can convince the programs of your sincere interest and do well your first 2 years of medschool and on the GMAT.
As for motivation, it opens economic avenues of healthcare policy which I am interested in where as an MPH would not do this. I am interested in the economics behind escalating healthcare costs and medical malpractice. Obviously, I am more qualified for an MBA than an MPH as well, another factor. If you are interested, the MD/MBA also opens the avenue of entering managed care on an executive level, but I am anti-managed care so no interest for me there.
As for the medschools that have good programs with respected MBAs:
Columbia
Duke
Michigan
UPENN
Northwestern
UCLA
Vanderbilt
The business community basically considers MBAs meaningless if they are not from a top MBA program, unlike the MD degree. I am unsure how the lesser MBA programs with MDs are perceived. IMHO, I think they are looked more highly upon just do to the rigor you put yourself under by taking on the process.
Baylor
Tufts
UCD
UCI