For older, 1990's era Honda Civics, I might use some made-in-Red-China parts. But not on my W220's. No way.
Since you're in the UK, and I'm in the States, availability and pricing of the below may vary.
I use Mann engine air filters (US $15) and fleece oil filters (US $12) in my cars. Apparently Mann is one of the OEM's for MB, and they're very reasonably priced, so I go that way.
For suspension components, I've gone with Lemfoerder, since I've been able to confirm that they are one of the OEM's for MB. TRW is apparently one of the other OEM's. For air springs, I use Arnott remans, due to their good reputation and affordable price. However, I've heard that shipping to the UK for Arnotts might be cost-prohibitive.
For your car, I might also suggest a transmission fluid and filter exchange. Yours being a 1999, is that the year that has the torque convertor drain plug? Seems like, if true, then draining out the old fluid would be even a bit easier. Anyway, a Genuine Mercedes transmission filter is only about US $15, and that's very affordable. Same for a new transmission pan rubber gasket, about US $16. New ATF, I've routinely gotten 236.10-spec transmission fluid on eBay for about US $8.00 and change per liter, including shipping.
If your engine air filters looked like that, then perhaps your cabin filters, both the paper and the charcoal ones, could stand to be replaced as well. I've seen some *nasty* stuff in the paper cabin filters; those are the ones you access through the glove box.
Also have a look at your driveshaft flex discs. Any cracks, good idea to replace 'em. Currently I'm using Febi-Bilstein flex discs. These are made in Germany, so I'm guessing they'll hold up pretty well.
Oh, and your transmission mount, too.
Those are about US $30 for OEM (Corteco or Lemfoerder).
I did the valve cover gaskets on some S430's, S500's, and one S600 TT. For the valve cover gaskets themselves, I have learned the following:
*DO NOT* use RTV or any other similar "sealant"! You might get lucky and not leak, but as I found out the hard way, you certainly increase the odds. It's not fun having to redo it and get that gunk off of an M113 engine, let alone my M275! Every car mechanic I've spoken with has--with a chuckle--confirmed this, unless the factory itself does so (e. g. a *small* dab of HondaBond in the corners *on Honda vehicles*). Save that RTV or similar for the evaporator covers.