Don't limit yourself to an old Disco 1 axle as even the newest 300TDi one (the type you want) will be 18-years old so could be a worn out heap of rusty steel !
I bought a Disco one for my 1986 90 back in 2004 and after a tidy up (new stub-axles, bearings, discs, calipers, paint-job, fitting of the 24-spline diff & halfshafts out of my original axle) I bolted it on and had 11 years excellent service.
Fast forward to last year and it was starting to show its age (the pan was corroding through and it needed another paint-job) so rather than messing about I picked up a 2-year old TDCi rear axle and bolted that straight on (the only difference was that I needed different bushes for the lower shock mountings).