I don't think the miles matter as much as the condition and recent repairs. A 70,000 mile car that needs variators, subframe, louder exhaust, lambdas, bushes, arms, discs, calliper-painting, concave grille, etc, is worth £8k less than a 140,000 car with all that recently done, IMHO.
Regarding breaking, the parts are really valuable, so I think it is all about the labour, and the attitude to used parts. If I was mechanically-minded and had a reasonable-sized garage, I would definitely snap up a cheap sacrificial QP, and gradually take off what I needed to keep my 'real' QP going. But would not want to have to actually break the car and then try to sell each part on eBay with the inevitable hassle when some parts failed soon after sale. YMMV
Last time I checked a front light unit was £1,700.