you will end up with 225 x 40 x 18" if you went to 19" you would virtually have no sidewall to speak of at all, as you increase the diameter, you decrease the profile (height) then your speedo still works
Yes you can, as long as you don't exceed the minimum wheel offset allowable on your W202 (My research tells me front offset 35mm, rear offset 32mm), and taking into account which tyre size and make you'll be using.
I know for sure a 19X8 or 19X8.5 with 235/35/19 all round will fit your car if the minimum offset mentioned above is adhered to.
Certain tyres like the Michelin PS2/Continental extra load version, as well as the Pirelli P-Zero range have very full profiled sidewalls that can potentially catch your wheel arch lining/arches. Other tyres such as Falken FK-452s have traditonally very narrow side walls and they will give you more clearance.