Nuru,
Mowing at the highest level? There are a couple of ways of interpreting that. Like Trescows suggested, the leveling wheels may not be set correctly and the MMM deck is floating and not making ground contact. Did the owner mow with the deck at it topmost draft level?
When setting up the deck for mowing, I believe the owners manual suggests that you keep the wheels about a half inch off of a level surface (like your garage floor). This seems to work very well for me. You're going to have some vibration. Heck you have the engine running at 2100 or so rpm, the MMM PTO engaged, 3 blades whirling around, and the tractor moving. /w3tcompact/icons/crazy.gif/w3tcompact/icons/smile.gif
I guarantee that after you spend a couple of hours mowing, you will feel the vibrations continueing after you climb off of the tractor. Just like in the cartoons.... /w3tcompact/icons/laugh.gif/w3tcompact/icons/wink.gif So, maybe the previous owner is little oversensative.
One of the things that you have not mentioned is if the owner is going to ship all of the MMM linkage. I can't remember if it comes with the MMM or it is a separate item (knowing JD, it's probably separate!!). Without that linkage system, the MMM is worthless and unusable. There are the main lift links and a set of brackets which work in conjuction with the 3pt hitch lower lift arms. Just a word of caution so that you do not have to use any of that fine Navy language.....
Terry