Just a word from experience: Make absolutely sure your wife is going to use it and is going to stick with it, because everyone I know that has bought a treadmill uses it for about 2 weeks then never again. It turns out to be a waste of money and those things aren't cheap.
FYI, I find a stationary bike much funner and easier to stick with. It's just a matter of preference though, committment is the main thing. Good luck.
Also, try checking classified ads, I bet you can find a used, good brand very cheap because people stop using them as I mentioned above.