My youngest has a Cat D Alfa Giulietta. Bought as such, from an Alfa specialist who rebuilt it following a frontal deer strike. Lovely car that drives well and has been no trouble over the past 5 years. (Hope that's not the kiss of death!)
So for me, generally no problem, as long as it's advertised and sold appropriately. As long as you trust the repair/er then it should be a good deal, but that discount will follow through and may well make it harder to sell in the future. This Aero +8 is however a different matter due to it's construction complexity and performance. Caveat Emptor is the name of the game.
I agree if you knowingly purchase a Cat D and both parties are happy it can be a good solution, I am puzzled why this one was not declared as such in the advert and how on earth can it have an agreed value of 85k when the insurer will be in possession of the full facts.
So there was two reasons for not being interested in this car and imagine others will arrive at the same conclusion when they call up for an insurance quotation.