Whilst the car might be rare in terms of physical numbers produced, this does not necessarily translate into increased value. Lots of cars have been produced in very limited numbers but it takes more than just limited production volumes to increase value.
AMG does not really have much pedigree from that era in terms of pedigree or motorsport involvement for these cars which would uplift value. Furthermore, the Estate tends to have a more utilitarian outlook/image which deflates rather than increases value. If he was really offered £8.5K for it , he was a fool not to take it.
I agree that rare does not always equal expensive and that the estates are not as sexy as the coupes or cabs which is reflected in the AMG production numbers. However, if you are in the Market for a petrol 124 estate wouldn't you prefer one with an AMG engine? Would you not pay a premium for that? The size of that premium is the debate and will be a decision for the individual. Fuel economy (or lack of..) will put many off, but they will seek out a diesel.