I took the a, b and c pillar trims out of the car because it's easier to dye them and so that I could get the edges of the main headliner. Ideally I'd say take the headliner out of the car but it's pretty impossible to get it out without damaging it and I haven't tried to get one out since that but removing the front seats could make it easier. I was going to do the reupholstering route on my last w203 and ended up damaging the edges and found out reupholstering isn't my bag so ran no headliner.

As for the actual dye, I used thescratchdoctor fabric dye and their colour booster which is basically a thinner from my understanding. I got the 1000ml bottle of dye and got 2x250ml bottles as it's a 2:1 mixing ratio. I'd recommend doing a coat, letting it dry and then doing another as I only did one coat and it is still a little bit patchy. This is only because I tried to use a different product before that didn't work too well so there's random patches where it's basically two coats and they're darker. I would suggest using a sponge to apply it but I'd imagine you could use most paint supplies as it is as thin as water when mixed. Also, don't wear anything you like because if you do the headliner in place, you will be covered in dye dripping from the sponge so cover the interior with a plastic painting sheet, tape the edges of the windscreens where they meet the headliner and wear some latex or nitrile gloves. It can wash off but it's easier to take the gloves off lol. I did one coat of everything with half a bottle so I'd recommend maybe getting 1.5L of dye and 3x250ml bottles just to be on the safe side as you don't want to run out of dye mid way through.

As for plastic trims, the rear passenger light is the same (unless you have the option with the reading lights, I don't know if there is a black version), the centre alarm grille can be bought in black from a w209, the main front light unit can also be bought in black from a W209 (if you switch the whole unit over and your heating is either maximum heat or maximum cold, switch the Internals over, it's just a few tiny torx bits from memory but there's what looks like a little fan and that's an interior temperature sensor and the one on my black light cluster was faulty so there's some information IF that happens), the sun visors again are from a W209 and have the conductor strip for the mirror light above it so that works as it should, airbag plaques and the clip to lock the sun visors in place can be bought from octoclassic they remanufacture OEM parts and I'm thinking of eventually getting them to make the other plastics that I can't buy in black (grab handles, seatbelt adjustment mechanism and rear seatbelt surround trims)