I have been thinking about high mileage cars lately. When I was younger, 100,000 miles meant that a car was one foot in the grave. Now, 100,000 miles is about the entry point for a used car. So what does high mileage do to a car?
Here in PA, rust is the first issue. High mileage cars are usually a few years older and about to have serious bodywork issues. But if you are in a more temperate climate and the owner has been good about maintaining the car, the materials and protective measures of modern automobiles mean that the body might not be any issue.
The second is wear and tear from so many miles. But really, what wears? You replace tires and brakes all the time. You should do belts and hoses, along with plugs and wires on a regularly scheduled basis. That leaves shocks, springs and suspension bushings, which you can also replace. So as long as the wear items are replaced, then the car should be fine regardless of miles. (I am making an assumption that metal fatigue is not significantly going to affect a-arms, mounting points and such on a modern car with under 1,000,000 miles)
The third item is of course the engine: there are parts that will wear over time like bearings and cylinder bores and piston rings. If a car was designed to go 100,000 miles and still be under warranty, then the engine must be built for durability. Modern materials again make mileage less worrysome. The best bet here is meticulous maintenance and oil changes. I'd also like a non-turbo and low-revving engine for longevity.
The final item is component failure. How long will window-wiper motors and door locks and water pumps last? These are all kind of a gamble. But if you have some money in a fund to cover the failures that are sure to come, then it might not be that bad.
So my theory is that a modern car with modern materials that has been properly maintained could easily start a new life at 250,000 miles, as long as you budget around 25% of the purchase price for updates immediately, and another 25% for maintenance and repair over the first year with the car.