I would assume the age, flight hours and number of takeoff/landing cycles for any aircraft is what correlates with the amount of maintenance required, far more than which type of plane it is, among any of the current commercial aircraft being flown. If one type of aircraft was an outlier in this regard, it would soon be phased out of service and nobody would buy them.

I am susceptible to motion sickness as well and I think the MD-80 is fine. The wild card is the weather/air as you can experience motion sickness on any size plane, but bigger is better (I have had a very few bad rides on 767's and 777's). Take some Bonine the day of the trip. It does not make you as drowsy as Dramamine. The bracelets are worthless (IMHO).

The MD-80's are a lot more quiet in my experience. By the way, Mythbusters did find ginger to be effective against flight sickness. If she doesn't like small planes though, might be easier to book on a bigger one. Not that many MD-80's around.

Bonine (meclizine), aka Dramamine II, knocks me out in a few minutes. YMMV :)

My wife has never been troubled by the severe motion sickness she used to suffer since she got a Relief Band. We got one with a replaceable battery, which is unfortunately no longer available. Relief Bands are registered medical devices which work very well for many people. I've loaned it to a colleague whose wife also suffers, and it worked perfectly for her, too. Two out of two successes in my not-so-vast experience!

MD-80 is small? Naah, an Embraer Brasilia EMB-120, Pilatus PC-12 or a Beech 19 is small. Since the wing loading, max take-off weight, # of passengers etc. are pretty comparable between the MD-80 and the 737 (and both have a lot of variants) the ride will be about the same. My advise would be to travel early in the day when the bumps will be less, give her a 1/2 a Valium and enjoy the ride.