That car is genuine AMG modified with the stage 2 engine modification, which consisted of high-lift AMG camshafts, ported and gas-flowed heads, to give a 55bhp increase to the base 231bhp of the 5.0 V8 M117. The quoted power figures seem to vary, some official brochures say 276bhp, the on ei have from Stratton say 55bhp increase, which implies 286bhp. The 55bhp may well be total output per litre, which equates to 275bhp.
Still a good car and in my opinion worth more - AMG's widebody press cars were white and this one has the AMG interior door cars as well.
I'd price it at 15k rather than 12.
Widebody's with a DOHC engine are worth considerably more, circa 25k gbp if they are in concours shape.
talbir