I just finished replacing the oil cooler lines on my '97 D90 NAS, and documented step-by-step process in the link below. Hopefully, this doc can assist others in deciding the difficulty and whether this is a job they want to DIY or pay someone else.
In total, it took me about 3hrs... just because I'm about 3 weeks new to my Defender and still learning. If I were to do this again, it should take me about 1/2 that time. Enjoy!
DIY on Replacing Defender 90 Oil Cooler Lines