The previous gen Focus rims won't fit. The Bolt pattern is different. Find a good tire shop that sells used rims. The bolt pattern of some previous gen Lincoln, Ford, Volvo, and Jaguar fit the MKIII Focus (5X108). I used Volvo (S40) rims for my winter wheels.
I wouldn't worry about the 16's vs 17's unless you are planning on upgrading the rotors to ones that won't fit on a 16 inch wheel. If you are going to go through rubber, smaller wheels are better. They should be lighter too. If you pick a good tire, I wouldn't worry.