Trying to value one of these beasties

I have a opportunity to get a GTA saloon but has had engine problems

So the Q to you guys and girls who own this model,is what would you value it at?

Car is in good order and undamaged except the engine issue

Interior is all black,multispoke alloys,xenons,what i would describe as denim blue paintwork

Oh and no service history to hand,so that in itself is gonna take a chunk out the value

Your views?

Derek