I have an '05 HSE with Nav and HD package. The good news is that I don't think there's typically much of a premium for HD on the used market. Bad news: there are fewer, so it requires more patience. As others have said, it's really very capable with or without - although I don't know about the "without." I have heard it said that the rear locker just makes it a little less dramatic getting through some of the steep / rocky / tough sections.
Re Nav, I sometimes use offroad maps, but it's not my primary navigation. My tablet with ViewRanger app is much better. I use the offroad display more. It's mostly for interest (hm: wonder if the rear locker is engaging?). The only time it's really been helpful was for diagnosing and alerting me to some suspension anomalies, where the height sensor goes wonky and thinks a wheel is down when it's not. The IID tool confirms this, and could provide the same info (in more detail), but I have the old IID display through the dash center, so it would take a while to plug in the unit and drill down to that data.
Re maintenance, knock on wood, but I bought mine used at 90k (with good records and good upkeep) and I've had good results. I've continued preventative maintenance and have done a few fixes myself (replaced a height sensor, replaced both front air suspension shocks when one developed an intermittent leak). There's lots of good DIY procedures on various forums. I think LR3s reliability reputation suffers from sins of previous Land Rovers. Just this past week I was talking to a friend who sold his 03 Range Rover, and he exclaimed that he hated me when I told him about cross country highway trips and multiple overland trips without incident on my '05 with 125k.
Good luck!