Watch the rear hatch for rust, that is one of the most common issues with the RRC. I know you can get Fuel Injector kits for them, i wouldnt worry too much about it as fuel injectors are better IMO than carbs, but the 3.9 and 3.5 are very similar as they are all a 40+ year old Buick aluminun V8 design, many vehicles owned by Rover used the motor. That seems like a good price, sounds like you got a good deal and suspension works nicely. Good luck.
I know seats from Disco 1's fit in the Defender, so you may be able to get a set from a Disco 1, but dont hold me too that.
Just Found this on Wikipedia:
"Petrol-fuelled Range Rovers were fitted with carburettors until 1986, when they were replaced by Lucas electronic fuel injection,[8] improving both performance and fuel economy. Some export markets retained carburettors, with the original Zenith/Stromberg manufactured units being replaced by Skinners Union (SU)-manufactured items."