I suspect that this is something you were told over the phone but which never materialised in writing, am I right? Then I'm afraid there is going to be little you can do but make your point firmly in writing to them. It was probably some call-centre operator trying to jack up his sales in the...
It'd come under Cat B (Motor vehicles with a Maximum Authorised Mass of up to 3,500 kg, no more than eight passenger seats, with or without a trailer - weighing no more than 750 kg), the same as a "conventional" car.
Yes, you may possibly need to hunt for insurance, it depends on the company...
As Charles says, it will cope with it fine with a few basic precautions. I would have said drive it fairly gently, but it sounds like you do anyway. You may get bored, so throw in a few breaks for both yourself and the car.
In fact, it may do the car good if it's been dragged around in traffic...
That would be about the right price for fitting a brand new Fiat part. They are available remanufactured for something like £300 + from a reputable source. If you search on line you'll find plenty as they're notorious for failing and there is a goodly supply of duff ones ready for the...
You'll need to check the specs, my friend. It should say whether it will run on either voltage and whether there's a switch you have to move to accommodate this. You may also have to obtain an adaptor to change the American-style plug to the UK style.
It can be, but that all depends on what the parts are and who you get to do the work. If you go to a Merc dealer it WILL cost you, but there are many independents, often one-man-bands, who will look after you much better.
Generally, in fact, elderly Mercs are more reliable than anything French...