The engine is actually one of the sturdiest engines around and with the exception of general wear and tear on high mileage cars, the engine is not one of the cars weakest points.
If possible have the car started from cold so you can look for smoke on start up which is evidence of valve guide wear on the valve gear. This is not a serious problem but the cylinder head needs to be removed to correct this.
When the engine is running and up to temperature the noise from the top end of the engine is normal in fact if there is no noise then the rockers probably have been over tightened, perhaps to hide wear. But doing this will accelerate the wear in the valve gear.
Any rumbling from the bottom end, or block, means a worn crank and main bearings so walk away unless you are prepared to do an engine rebuild. A chattering timing chain is a sign of wear, but fortunately timing chains can be replaced fairly quickly, inexpensively and with the engine still in the car.
The presence of water in the oil is a sure sign of a blown head gasket and potentially a cracked or warped head.
Oil leaks are commonly found near the timing chain cover, gearbox bell housing cover, drain hole, and tappet side covers
It is difficult to measure the oil pressure as most Spitfires do not have a gauge fitted so you are relying on the low pressure warning light (when this comes on it is already too late). At start up it will take a few seconds before the light will go out and you might hear so rumblings from the bottom end, this is due to the fact that all the oil has drained away from the filter and has to be pumped up again. Running hot indicates a worn water pump, a blocked radiator, or even burned exhaust valves.