Not a huge level of experience on this one but as no ones answered so far I'd say warranty direct, warranty wise etc. I've got RAC warranty never claimed.
You may find its cheaper not to bother with one and foot repairs yourself although on a car like yours it wasn't the case but they weren't all basket case sheds so I'd not bother as yours is young.
I had one. It's horrific unreliability and poor transmission was why I got rid of it? If you find 7g ok (I hate it) you'll love the car. S class refinement, near c class sport handling. E220cdi fuel consumption. A marvellous package and feel good factor.
If yours has main dealer service history you can get MBs tier 1 warranty, which is water tight.
Honestly though, save up £1200/yr for repairs and you'll probably never touch it and have a wee money stash at the end. Or have the chance of.