Joe,
Eric is right on all aspects. My 84 has quite a few differences between itself and a NAS Defender when you get down to it. Dimensions are the same but body panels, tank location, light arrangements, etc. are quite different. Unlike the bar "bulkhead" that is in the NAS, I have a big steel bulkhead right behind the seats so you're not able to lean the seats back very far. My gas tank is located under the passenger seat with the filler spout just behind the door (see picture in my profile) as opposed to NAS tanks under the cargo area. It is a factory LHD unit but there are lots of differences though.
The truck currently has a 2.5L n/a diesel engine but I will most likely switch it out and put a 3.0L 5.0 cylinder MB turbo diesel in that drops in quite nicely.
As far as locating parts for an 84, I haven't had too many issues. You can find most of what you need on ebay.uk or flip through any LR magazine and you see many suppliers. Britpart carries most of what I need but even here in Canada, I have local contacts that I can get parts from if I need or you improvise and come up with a different solution. This is if you aren't a stickler about using OEM parts for everything which isn't always the best to begin with. I've also noticed that some parts for ex-MOD Defenders seem to be cheaper than their counter-part civilian versions...at least on the parts that I've looked for.
Park a NAS D90 next to an older LR90 and you'll see a lot of differences. Overall they resemble each other but details-wise they're different.