Low mileage doesn't help the age related problems, like seals degrading. Then there's actual use; a 20 year old car with under 100k miles hasn't had much use, so it's spent lots of time standing. And that causes its own problems, especially on complicated cars that are already fussy to maintain. Old, low mileage cars have often had normal maintenance delayed because it's only done 1500 miles since the last time you looked at it.
If you want a toy, or are prepared to quickly do all the jobs we know will be necessary then none of this matters.
But anyone expecting to have a daily driver from the moment they buy it needs to looking at cars that are already in daily use. I would rather buy Amba's high mileage Omega than Lizzie's much lower one for that very reason.
My Omega is in daily use. But, I know what you mean: high mileage cars that are started before doing longish journey's every day have the oil circulated around a hot engine for long periods, the exhaust is brought to, and maintained for a long time, at a high temperature, so any harmful moisture is constantly dissipated.
So, yes, a high mileage car can actually last longer. However, for 25 years I had brand new fully expensed company cars that I regularly drove 100K+ miles in two years, one even going to 108k in one year. As usual those vehicles, fully maintained, were changed after two years with another brand new vehicle. The company did that because after that level of mileage the maintenance costs would rise with parts wearing out, let alone an MOT being required, and the trade in value of the car would depreciate quickly after the two year period, on a vehicle with costs escalating. Now those cars were of an age where after that level of mileage the engines, especially on my Ford Cortina's, would show severe signs of wear, even with great maintenance. It again was good business sense to replace.
So, in the case of high mileage cars I would argue that, along with wear and tear on the bodywork, can be a greater risk than (some) low mileage cars like mine that was / is maintained to a high standard and kept overnight in a garage. So, recognising you mention Nick the "under 100k miles" rule, in fact you can reverse that to saying "over 100k" can mean you face ever greater bills (like on my company cars when then in private ownership), and over 150k they can get extortionate. Like my 150k Senator I had in private ownership, it needed a new fuel tank due to rusting, the gear box requiring repairs, new oil cooler and pipes, and then severe (near terminal) rusting around the rear wheel arches, which made me trade it in for my first privately owned Omega 3.0 Elite. Therefore I calculate that with my current 80k mileage Omega, with the correct maintenance being undertaken, can reasonably be expected to last to the 125-150k mileage before costs become extortionate.
Right or wrong, it should get me at least 10 years future Omega driving before I will probably give up motoring, or at least been able to obtain a non-fossil fuel car (with a Government grant!

) that drives me!!

For me that all seems good, but yes if I was going to buy another Omega now, IF I could find one 8 years down the line and knowing I would never again have it at 54k miles on the clock, look at what parts had been replaced, the level of professional or semi-professional servicing,
before looking at the mileage. 