The engine used in the Lotus Carlton/Omega is based around the C30SE engine as used in the 24v Senator and Carlton/Omega-A. The unit is fundamentally strong with one significant potential weakness - the timing chains. There are several cars that are known to have in excess of 100,000 miles (160,000 kms) on the clock without a failure. Experience has shown that the chains will more than likely fail within 65,000 miles (100,000 kms) with normally disastrous consequence. Check for timing chain rattle or whine from the front engine, don't confuse any noise with the ticking from the fuel injectors.
In my opinion DO NOT go near a car that has been chipped or modifed in any way. Chipping has been shown to affect the smoothness of the engine and can cause, head gasket failure, piston failure, engine management faults and in extreme cases, broken drive shafts. You have been warned.
Look for blue smoke in the exhaust that can indicate worn valve guides/oil stem seals, tired turbochargers or the cats being removed.
Check the expansion tank for mixing of oil and water indicating head gasket failure.
Ask what oil has been used in the car, the specified fluid is a competition grade mineral oil (20W/50). Be wary of a vehicle that has been run on low-viscosity oil.
Check the oil level from the dipstick - low oil could give an indication as to how the car has been cared for.