The tyres are not too difficult to get - just expensive ! These are cross plys - not exactly the best to prevent tram-lining. The "welting" is the fabric that goes between the inner and outer guards to act as an anti-squeak. Also sort of prevents water getting in between the panels, but is not so effective at that. It should always be body colour. Same for the inside of the mudguards. It looks like the silver one has been painted black, but it may be a shadow.
The difference between the 300 and the 300b (and c) is the front window. There is no quarter window. The hubcaos on mine are a one piece incorporating the wheel trim, whereas the 300 has just the hubcap. On the b, these one piece units cannot be rechromed as it requires a special anode to draw down the chrome into the curved part of the trim - and that is if the chrome plater could even get in there to polish it. They cannot be taken apart satisfactorily as the cap is rolled onto the trim. MB list price for these is in excess of US$500 EACH !
Yes, it was me telling the story about the Classic Centre, and it was the licence plate lights which were upside down.
Also, notice the two chrome strips above and below the 300 badge. These are almost always missing as the clips rust through and they fall off. I notice that the one Des is looking at has one, but it seems the bottom one is missing. Replacements are available for about EUR 100 each - unchromed ! The good part about all the chrome work is that it is on a brass base, so rust is not a problem. What is more difficult is if a piece of chrome is missing, as each piece was fitted to the car, and then plated, so each one is unique.