You are braver than me Dammit, taking an old C55 to a garage and telling them to 'fix everything they find..' ! How much is it worth ? Only what someone will pay for it is the glib answer.
Some on here say 'why get an old V8 when a diesel car will give it a "good run for it's money.." , not sure what they mean by that, diesel MB of the same era with OM642 V6 had a tiny bit more torque and were more than 100bhp down on a C55. Add to the fact that
nothing sounds as good as an N/A petrol V8 and I have yet to see a diesel car with any character, or one with an exhaust note anyone wanted to hear.
Fuel consumption ? who cares ? life is too short. The price of tidy C63's are firming up not because they are destined to become classics but (IMO) some people just want to drive the last of the N/A V8's before we are all wrapped in plastic 'cars' fitted with washing machine motors that have been designed by politics steered by dodgy science and as a result possess and a crushing lack of charisma .
If the 'plan' to tax these cars off the road in the near future ever goes ahead they will be worth whatever scrap cars go for these days £40- £60 , or a track day car that needs a manual gearbox fitted to make it fun, who knows ?
I am not sure if it is that I have simply had much better luck than you with my 147000 2006 C55 as my running costs have been negligible and it's my daily drive (a little less now in lock-down) , I also do a lot of the maintenance on it myself.
This .