Most cost effective is painting the interior. I've been in touch with Vipond Paints in Australia, I got this tip from Benzworld. SLK's have the same problem we do with flaking interior paint. Vipond sent me a brush out of their SLK interior paints. These match the factory colours and are waterbased, rubberized paints that have the same sheen as the original. Their brand is called Tautflex and it can be brushed or sprayed. Anthracite/charcoal appears to match our SRT interior. Price is $45 shipped for 250ml and I'm told that's enough to do the SLK dash and console 3x's. They also have paint that will adhere to rubber, in particular a silver that will match the Infinity radio *****. I've also been in touch Parasol here in Canada. They too provide interior paints and leather dyes and will custom blend colours. Parasol is well regarded by the local Jag and Merc and classic car restorers and boating communities. They also sell high end detailing products.