Underbody corroded. Now there's a phrase that covers a multitude of sins and I certainly wouldn't expect an MOT advisory if it only needs resealing.
As it stands I don't think the low mileage is a factor it's just an MOT failure that's been off the road for over three years and will require recommissioning as well as MOT repairs so it's spares or repair really. I'm prepared to stick my head over the parapet FeaRThiS and say your looking at £250-350.
On the road with an MOT however, the low mileage is a good selling point and it's a good looking coupe, so you could be looking at over a grand but with the amount of work involved to get it to that stage you probably won't be looking at much of a return.
