As a Land Rover owner before there was a Land Rover North America (It was actually Range Rover North America, the name change was in 1994 when they imported the Discovery and the Defenders) I say, go for it!
I have bought all my vehicles new and there are 4 in my driveway at the moment, but I LOVE them. Yea, gas millage sucks. But if you're cruising on the highway, you can squeeze 18mpg out of them (and I'm talking about the old Buick 215 based engine!).
There isn't much in the way of differences when it comes to LR3 or LR4. So if you really want to update your LR3, LR4 lights are a direct fit.
Avoid the V6. It wasn't a bad engine, it actually gets WORSE fuel econ than the V8. Why? Because you end up flooring it all the time to get on the highway, pass someone, keep up wth traffic, etc. That any fuel savings is negated!
The Range Rover Sport uses the same engine and chassis. Technically, and I would love to, a supercharger should bolt right on! Find the parts on ebay a mechanic with the land Rover "Test Book" computer and your're good!.
Overall, I have NOT had many issues with mine, and I drive a 2000! The older it gets, the better it seems!
My mother has a newer vehicle, my sister one of the last "Disco 1s" and my father and I drive the Disco II.
Call me crazy, but i feel they have a personality, they are wonderful vehicles. And although part of me wants to upgrade to an LR3/LR4 (called Discovery 3/4 everywhere else in the world!), I love my 13 year old Disco II!
What I do miss is the simplicity of the Defender. I sold mine 7yrs ago. My wifew sometimes hated it (noisy, difficult to get into, etc) but at times, loved it as well. And unlike most girls, she can drive a stick! So I let her drive it once or twice.
Honestly, having owned more Land ROvers for longer than your average Rover owner... I say get it... JUST GET A HISTORY!
The new Jag/Rover V8s are great. Quality...well...maybe I'm brainwashed? but the quality has improved over the years. NO ONE has had a major issue. Personally, I think they are vehicles to keep. But you'll become attached to it anyway!
In my Disco 1, I used to drive off road. The 1st year (2000) that I owned this Discovery, I went off road a few times, but it's basically just stayed on the pavement!
Other than the Tires (moved to a more aggressive and slightly large size) it was STOCK. And I can show you with pictures of us (my friend and I) pulling Hummers (H1s) and Jeeps out of "situations."
When I was living in Maryland, Land Rover NA was in Lanham, MD. So knew some people at there and used their private off road course a bunch of times.
Anyway... I think you'll love your vehicle (provided it was treated/maintained properly before you).
But the days of people making Lucas & the Prince of Darkness and British wiring/electrics and build quality jokes are gone.