I have had no problems with my 05. I would try to stay away from 04, but many have had no problems with there 04's either. I would look for an 06 or 07. 07 is when they started the VVT motors, which gives them a bit more power on the top-end. If your getting an early year Titan. Ask whether it has had the brake judder issue and any problems with the rear axle seals and differential. The newer you go, the less these problems had effect. But heck, I've seen 07-08's going for 15K around here with 20K miles or less
Alot of the '05's are also problem free. A larger intake manifold and VVT was introduced in the '07's and I think the 2007 and newer might possibly be getting a little better MPG than the 2004-2006 models.