Chris, my advice is this......test one first, the gearbox is horrific driving slow, mpg is very very bad, sounds like a diesel quick cars when you stoke them up drive them slow and not nice. Get a sports exhaust and flat out they sound amazing tubi rumour but be aware its very very loud!
Another tip get a 2005 if u must have one and pay cheaper tax 2006 on and its in the £450 bracket as is all M6's
In all honesty with 15k i really dont know what i'd choose now having had most of the budget supercars. Audi rs6 is nice at 8k. Sl55 amg again nice at around 15k. If honest my S600 at 6k would wipe the floor with the lot on straight line performance. Difficult choice. Maybe an M3 CSL if cheap enuff as i'd more look for holding value.