With the Rover V8 it is never 'how many miles they have done' but 'how they did them'.
It is not uncommon to see contamination on a ten-year-old or older engine that has covered typically less than 50K miles and has had only factory specified interval oil changes, but please don't let this put you off. A V8 that does a thousand miles a week would last perhaps 200K miles and still be in great shape.
Almost all used Rover V8 engines that have covered in excess of 70,000 miles will be suffering, or beginning to suffer, from worn camshaft, lifters and a stretched timing chain. This is normal with the V8 because these engines are amongst the most forgiving of engines.
The result is a gradual decrease of engine efficiency and economy that goes quite unnoticed to start with. This is due to the fact that although you have eight cylinders, the major loss of efficiency is only in one or two of them (typical of a worn camshaft). This may not be apparent until they have perhaps failed completely as the remaining good cylinders will mask the power loss effectively until severe enough to make replacement unavoidable.
However, if left this long many other engine components will have suffered due to this and can lead to an engines condition being far too bad for an inexpensive overhaul.
Why unnoticed? With the hydraulic tappets taking up the clearance as things wear away, by the time you get to hear them, they are already very worn. The initial problem will be a gradual loss of power over a few thousand miles. Also, you will notice the need for changes in carburretor and ignition settings to cope with the excessive emissions produced, seemingly from the engine as a whole but more likely the individual cylinders that are suffering the most camshaft wear, causing loss of full or correct valve lift.
This change of emissions (High CO and HC) is why most mechanics would be incorrect when they advise that your carburetion or ignition is in need of attention, when in fact the problem is more likely to be camshaft wear, along with erratic ignition and cam timing due to the timing chain stretch.