Like most Carltons, the interior is fairly robust if a little plain. Do not be too worried about scuffs and scrapes to any parts that are generic to the Carlton range as this are generally inexpensive and plentiful second-hand. Anything specific to the Lotus Carlton/Omega however, is silly money.
It is likely that the front seat squabs will be scuffed (especially on the driver's side), these can be easily re-coloured to hide light scratches (using a kit from a car upholstery specialist) or have new leather panels sewn in. Look for tears in the carpet and damage to the door panels. Also have a look at the wood trim on the doors and the gear stick surround.
The glovebox should have a leather pad along it's top edge with a metal plaque in grey showing the build number, make sure that this is present.
The Lotus Carlton/Omega was delivered with a set of driving mats that are made of the same material as the carpet and have dark grey piping. If you are a stickler for originality, make sure that they are there and in reasonable condition as replacements are over £140 each!!