If you get a 1998 Anniversary model, you get almost all of the comfort of an ES without the PITA sunroofs, (you'd still get aircon) or the electrical stuff like electric seats which can go wrong.
If you go for a D2, they are a lot more trouble electrically and mechanically, and the chassis rot. One of the first tools you'll need to buy will be a diagnostic tool.
I have one of each so I know that I prefer my D1 Anniversary despite the fact it needs the sills welding and new swivels.
As for D3/4 I wouldn't touch them with a barge pole as the electrical complications are myriad. I know many independent LR specialists who won't touch anything after a D2. So if you are wanting a D1 for the lack of complication I am completely with you on that. I'd get rid of my D2 tomorrow if it wasn't for the fact that my wife can't drive a manual any more. Despite the fact that I have fixed most of the many common problems.
A mate of mine bought a D3 recently, he loved it but was so scared of it going wrong that he got rid of it. and he is no amateur having built a jeep from a RR and having used a D1 commercial in anger doing MSA recovery for years.
If the diff lock doesn't seem to work it will probably just be seized due to lack of use. Not difficult to take the central console out, drill out the pop welds, take the mechanism off, put it in a vice and free it off, grease it up properly and put it back in.
The biggest pain in this job is putting new pop rivets in, but you don't have to do that, you could use self tappers or anything else that takes your fancy.
As for all the "does it have a service history, low mileage, blah de blah" too obvious to need stating really, I am sure you don't need telling that!
By the way, the D3/4 is so awful it has its own website(s)!
http://www.disco3.co.uk/forum/ is one
So don't be scared of the mechanicals, just have a good poke around with a screwdriver where there is any doubt as to condition of the bodywork. Chassis CAN rust on them as they can on any vehicle, especially the rear cross member.
Best of luck.