Hey Abdul,
CLK55 is going great. Can't believe I've had it 4+ years now, longest I've ever owned any car for!
£300 is fair for the bumper if in good nick - they are very hard to find as you know.
I personally would stick to OE filters as I can't think of any reason why to fit non-genuine ones. They flow plenty of air for the 5.4 M113 N/A. I think even the supercharged M113 AMGs use proper pleated paper filters as does the McLaren SLR AFAIK - so no performance advantage to be had. Probably better money to spend elsewhere on these cars - new MAF or a gearbox fluid change for example
Mobil 1 is the recommended oil for these cars. At around £5+vat a litre I think it's reasonably priced too, and you don't need to change it for another 2 years/10k miles