Since the OP asked specifically about the Disco3/LR3, I'm only going to address the D1/2 to say the LR3 is a quantum leap forward in reliability. Really, there is no comparison on that front.
You said your primary concern was the suspension, specifically the air bags. In general, the air bags themselves have shown to be quite reliable. Mine have 115K on the originals with no leaks or damage.
Your worries are more about other components of the air suspension. Specifically the compressor and dryer, and the valve blocks. These seem to be the biggest failure points in the suspension if not the entire truck. While the valve blocks are fairly cheap, a compressor is pretty expensive. Figure $800 for a replacement unit if you're doing the work (easy enough to do in the driveway, maybe 2 hrs if you have never done it before and don't have sliders in the way). There are rebuild kits available for the compressors that can cut that cost substantially.
The original compressors have been replaced with a new model. I always get them confused, but I think the original was a Hitatchi and the new one an HKS.
A failed compressor will bring the truck to the bump-stops. Assuming you aren't running a huge tire, you'll still be drivable but it will be a pretty miserable ride.
Other than compressors, the most likely issue you'd expect to encounter on an LR3 would be random computer glitches. Most often these won't stop the vehicle but may put it into a limp mode. More often, you can simply clear them and continue on. If you do get an LR3, consider a GAP IId tool your first upgrade. Aside from all the cool things it can do, it's most basic feature is to read and clear any fault codes.
The basic powertrains on these trucks are very robust. Failures are not unheard of, but certainly a rarity. The old days of head gaskets, overheating, oil leaks, etc are long gone. My truck's chassis is totally oil-free.
Some people have mentioned suspension bushings, particularly front A-arms. I don't consider this a failure item, more a wear item. They don't just 'fail' and suddenly leave you sitting, but they do wear faster than on a typical car. It's a very heavy 4wd truck, and it's hard on bushings. If you live in a high-corrosion area you'll likely find the control arms are extremely difficult to get off when the times comes to replace the bushings (60-75K miles is common depending on how hard you use it) so a lot of people just cut them off and replace the whole control arm as an assembly. More expensive but easier.
It is a luxury vehicle, and the maintenance level and cost is higher than say a Ford or Toyota. But it's not ridiculous. And most of it you should be able to handle yourself if you're handy with a wrench and know your way around modern vehicles.
The short of it is the LR3 is a 'whole different Rover. Is it as reliable as a Camery? To be honest, no. But it's nothing like the older trucks. Don't be put off by an old reputation or the baying of the uneducated masses (particularly those Toyota drivers who tend to think anything other than a Toyota is a death trap waiting to happen...I have my own stories about crappy Toyota failures I could tell).