Not sure what you mean by a big difference between an 80's and a 90's Defender, but the biggest difference is in the engine, especially if you are considering a '97.
As you already know, a '97 NAS D90 will have an automatic transmission and an EFI engine, so right there is 2 major differences, otherwise they are the same truck.
If you are considering an ECR (East Coast Rover) D90, then get ready to spend a lot of money. What you'll have though is a nice sorted out vehicle, with practically nothing further needed except for some additional upgrades and accessories you might want to add in order to further customize your rig.
Thank you - Samer
1997 P38 Vitesse (Red)
2003 Discovery II
Defenderless, for now!