The Rapier is a special bodied version of the 127/130 defender, the load bed is 8 feet x 4 feet and it is intended to tow the variuous element of the rapier missile system, generators, lanchers, radar units etc whilst carrying the missiles in the back. the use of the 127 supercedes the rapier trailer made by GKN Sankey that I have, as trailers have fallen from favour with the MOD.
In some ways the Rapier Defender replaces the 101 in certain roles, as the 101 ofetn towed the Rapier system due to it's weight (Series and ealy N/A diesels being a bit overfaced).
As siad though, they are often underused and over serviced when demobbed so if you want a bargain V8 LR :D
Also look out for the 127 ambulance, same deal but an ideal camper conversion.
BTW the 3.5 V8 is restricted/detuned so will not have been thrashed too badly.