I got some 16 inch alloys off the bay for winter. You can get a set with summer tyres for a bit more than £100.
Viz the scene tax, Mercedes use 5x112, so some steelies from a C class should work.
For winter tyres, you might be better with 205/55r16 than 17 inchers, as you can fit normal snow chains. You have to get posh chains to fit on 17" or 18".