Now when you go to look at your very first iX. First thing is first, look at the front CV boots. Are they torn? If so, talk the price down alot... new shafts can be upwards of $1000 each, the cv joint could be fine, but its your chance to get the price down some more. If the boots are good, then you should be good to go on that part. The front diff is pretty hard to break, so dont worry about that one. Control arms and steering racks are iX only parts, so check them for wear and adjust price as needed.
Next, look at the tires, are they the same size? If they are not the same sized tires, walk away right there. There is a good chance the t-case will be damaged due to wheels turning at different speeds for a extended time. Wheels, for the iX only work on the iX, but 2wd wheels will work on the awd cars (offsets and what not)
You may want to check to see if the t-case works. In order to do that you need to jack up the rear of the car with a floor jack. Make sure your on a flat clear surface. Jack the car by the rear diff (with a block of wood) and slowly try to drive forward with just the front wheels on the ground. Here is a video to show you how. http://www.youtube.com/watch?v=a4exvt2_Yjc
Rust!! this is the leading cause of iX death.. since they are awd most people use them in northern areas that use salt. Common places that these thing will rust are:
front fenders under the wheel flares
front and rear jack pads
rear battery tray
rear corner of door jam
lisence plate lights
rear most part of pop out windows
That is about all I can think of right now, but would like to tell you right off the bat, a iX that needs work will cost you big time to repair. A iX that doesnt need any work will cost alot less over the years... you can catch any problems before they become a big issue.
I have copies of them somewhere..