fitting a disco transfer box

hi im back and needing more help, its a 1990 defender with a disco200tdi engine fitted, ive got an oil leak on my old transfer box that needs sorting, the question is should fit a disco transfer box or replace all seals on the old one ? i do all road driving, i have a disco box already, what else would i need and is it hard to do ? ive never done anything like this before, so any links or advise would be good
regards

depends on your tyre size and use,seals are easy enough to change

265/75/16s bfg a/t just fitted