I agree - seems a shame to have spent so much time getting it how you want it, to not enjoy it a bit - but I sort of know where you're coming from.
What do you have in mind for a replacement?
I can't recall the specifics of your car exactly (mileage/history/age etc) but I'm sure it'll be one of the best around. What makes this tricky is that there's quite a range of prices for CLK55s. I've seen some complete wrecks in the £5-6k region (rust all over, 120k+ miles, no extras (COMAND/Xenons/PDC etc), several outstanding faults, poor history etc) and likewise some silly prices on others (tatty private cars and £10k dealer cars, even LHD ones for well over £10k etc).
So I'd think you'd be looking towards the high-thousands, I'd guess maybe £8/9kish if there's genuinely no rust or bodywork issues, recently serviced, new tyres, no faults etc - which I think your car has
I've spent a fair chunk on mine recently - and I'm about to buy a new set of tyres (~ £500), so it's going to stand me in considerably more than what I paid for it, but I was aware of most of this at the time of purchase. On the +ve side, there should be little to no expenditure for the forseeable future - zero rust, fully refurbed wheels, new tyres, recently serviced, new discs/plugs etc.
Best of luck with the sale and naturally I'd be keen to hear how it all goes
Will