Also consider a 5 year straight finance deal without GFV as of course you have the option to hand it back after 50% term provided 50% of the money is paid which gives you a get out clause and most likely after 3 years you will have more flexibility to sell privately and retain the equity for your next car. Ultimately, it is true to say that some dealers will run from a used Maserati and offer low to reflect this but that doesn't mean that the knowledgeable guys are not out there, they are.
For example...
A £70,000 car with £10,000 deposit over 60 months straight would cost £1185.24 at a rate of 6.9%. You would have around £28,000 still to pay at the 3 year point but as bigbob mentioned, it is likely to be worth between £35 and £40K so ideally, you would still have your deposit swimming around in there somewhere. Worse case, you would lose the deposit too.
The other thing to look at is leasing which would reduce the amount you have to put in an give you a set monthly cost with no concerns after you term is up. Just give it back, end of story. Leasing is particularly useful if you can put it through a business as 50% of the VAT on payments is reclaimable. It's likely to be somewhere around £1400 a month but of course that's with a brand new car which is more valuable. You'd get a longer warranty too!
Finally, if you do decide on a c63, again look at leasing. I know these can be had for £650 or so a month plus vat with 6x£600 deposit. There's a reason for this and it's simply because Mercedes make too many, same as the rules for the M3. They build them, offer deals on them and pick them up after term for agreed reduced rates so that they can then retail them for decent profit. And so the world keep turning...