Spending £6,000 on a second car, then paying twice for VED, insurance, servicing, all to save wear and tear on an S210 worth - what - £4,000? I don't get it, I'm afraid, even if individual journeys in the smaller car do cost less in fuel. Yes, the friend who needs driving practice gets an easier car to practise in, but it would have to be a very good friend to merit that kind of expenditure.
No, it's not kind to the big car to drive it short distances, but it won't be kind to the small car either, and that - initially anyway - will be the more valuable asset.
So wouldn't it make more sense just to use the Mercedes, and give it the occasional gratuitous long run (maybe to Waitrose in Henley - they'll pay for your parking too) to blow out any cobwebs from trundling round the houses?