What Texas said! 138,000 miles is just getting broken in on an MDX (that's been taken care of, that is). Make sure that the belt changed was the timing belt, and not just the serpentine belt. It would also be helpful to know that the valves have been adjusted at around 100,000 miles. By far the weak link in an MDX is the transmission, so I recommend pulling the tranny fluid dipstick (the one that is down under the throttle body) and see what the fluid looks like. If it's clear and red, smile. If it's black and stinky... you have to assume that it's more likely you'll have transmission problems in the future.
Also, how good a deal that is depends somewhat on where you're at (and where the car is from). Cars from northeastern locations tend to be rusty at that age, while those from the southwest aren't. Also, anyone buying a used car right now should be very cautious about where it's from, as a whole lot of flood-damaged vehicles from the Houston area are making their way onto the market after passing through the insurance / auction process.
