I don't think your dealer is lying per se, but certainly miss-informed, and perhaps only nominally experienced in ********* off roading. The problem is that very few LR3's have the HD package. Was it available in 2005? Yes, probably more comon in 2005 models than any other year. However, I would recommend a 2007 model over a 2005 with HD if that became a deciding factor. I wrote an article on what I would want in a replacement LR3:
http://blog.nextstepdesigns.com/?p=78
Now, I have off roaded my LR3 more than most, and have spent literally months behind the wheel of many other LR3's, with considerable offroad time in them.
An LR3 without HD (rear locker) CAN go anywhere an HD model can go, but it requires more skill, more finesse, more effort and a certain amount of will. Having said that, there are very few actual situations where the Locker makes itself noticeable. In my experience, this is usually on steep accents or over very large boulders.
Having the HD package is a minor help in resell, if you are working with other off road minded folks, but I would not consider it a deal breaker unless the LR3 is going to be doing a lot of 4+ trail time.
I have a few thousand photos of what an unlocked LR3 can do on the trail here:
http://nwoods.smugmug.com/Land Rover/198186