There's no ECU on a 1994 Defender.. ;-)
If it's a 2 door, then it may be a tray etc , but if it's a wagon body, looks for rust under the body cappings. Their the ridgy bits half way up the body. Their not galvanised in a defender and repairing these is a pain in the bum as you have to take the roof and side panels off to access.
Rust in the bulk head around the hinges is my greatest worry with a Defender, fixing that sort of rust is painful as you have to strip the bulkhead and most of the vehicle to get at it to weld in patches. I've got a few land rovers parked up because of this, easier to go out and get another one than to repair.
Rust in the foot wells, well depending on how particular you are, can be fixed with replacement footwells with only minimal stripping. OR just a steel patch bolted or pop riveted in will make a reasonable fix.
The door frames rust, and new doors are expensive and hard to get. If you are handy with a welder you can get good results by taking the door off, deskinning it and welding in new frame sections. (My Wife did this as part of her panel beating course.. And she made a new door skin..)
The chassis can rust in many places, take a scree driver and look carefully, some Chassis repairs are not too bad, but unless you have a welder will be expensive.
When you do buy it, spray it all over with Inox Lanotec or fish oil or something else, especially in the inner cavities of the bulk head.
All of the other issues like drive train sloppiness, CV's, wheel bearings, worn gearboxes, etc are Maintenance on Defender and you should budget for this as maintenance, as you will need to service and repair these sorts of things during your ownership even if they are perfect when you get it.
