The car looks quite nice, and if it was refurbished 20 years ago, it held up quite well.
The trunk floor is superb. This is a known area for rust. If it's original, it's quite encouraging. If it was replaced, then you should expect some serious rust elsewhere on the chassis.
I don't know what is the issue with the back of the front right wheel well, but it doesn't match the condition of the trunk floor.
The seller is a mechanic and was a member of the MB club for a long time. He can't ignore the value of these cars on the UK market and the rarity of the factory 5 speed. There seem to be a slight mismatch between the described condition and equipment of the car and the asking price, given what it would be worth on the UK market.
I can't see well, but the engine bay seems to have been re-sprayed with some kind of undercoating. The side of the sills and trunk floor look really nice, indicating that they may have been replaced. So there was potentially a lot of work done on this car, including rust repairs. Nothing wrong with that, as long as it was done correctly. This is why you really need to have the car on a lift and have someone in the know inspect the floors, chassis rails etc. It is sometimes difficult to detect a poor patching job once covered by undercoating, but it may be possible to grasp some clues, like that front left wheel well for example.
Rust is really expensive to fix, so it will affect the value of the car.
Matching numbers is not as much of an issue on these MBs as it is on Jaguars for example. But if the engine was replaced, you have to make sure it was by the same one (displacement, compression ratio, camshaft etc.).
In terms of value, an original ZF 5 speed would be a big thing. And if it is, make sure the car came out of the factory with it. The data card will inform you on that. If the 5 speed option is there, it will be a real plus. If it is not there, then you will be sure the 5 speed is a later modification.