Avoid the early 2000 production models of the X5 4.4i. They were released to early and constantly have problems. The 4.4 V8 is definitely the way to go as the X5 is a big and heavy SUV (4,800lbs dry). The 3.0 liter is actually better than you think, though. It has good torque and will push you back in your seat and is a very smooth engine, but the 3.0 liter I6 is still kind of small for such a heavy vehicle.
Becareful when selecting options, ask your father if he prefers ride comfort or handling. An X5 without the Sport Package is very smooth and luxurious and drives like a typical BMW sedan. The Sport Package provides amazing handling, but at the cost of a significantly stiffer ride (mainly in the back seat...it is like riding in a school bus!). Personally, I would definitely go with the X5 4.4i Sport Package....I prefer performance over ride comfort and the Sport Package doesn't make things too bad up front.
Why not consider a new X3 3.0i? You'll get a brand-new BMW that does all the things the X5 does, but better. Plus, it actually offers decent interior room and a real trunk, unlike the X5. I bet it was the trunk space alone that made BMW decide to call it a "SAV"...the average mid-size SUV offers 32-35 cubic feet of cargo capacity in the trunk, whereas the X5 offers 16.1 cubic feet, which is a meer 3 cubic feet more than the VW Jetta's trunk. But then again, if your father doesn't care about trunk space and rear passenger room, the X5 will get the job done just fine.