Stick with the current tyre size on your current rims (225/45/17) and your snowsocks will fit fine, although how much use they'd see with winter tyres in place is debatable.
A lot depends on how long you intend to keep the car and what that translsates to in terms of tyres swapping costs versus separate rims.
If you allow £12 per rim to remove tyre, fit other tyre and balance then twice per year its ging to cost you £50 to swap tyres around. Do this for 2 or three years and you might as well buy a second set of rims
If you were happy to use 16" wheel n tyres for winter then there is additional saving on tyre costs between 205/55/16 an 225/45/17 (typically £40/50 per tyre)
So in effect you have three scenarios...
1. just swap tyres at £100/yr and £600 initial outlay for winter tyres in 225/45/17
2. buy 16" steel rims and winter tyres - typically £550-600 per set, and pay the garage 2 x £20 to swap wheels (or smile nicely and get it done for free)
3. buy used Mk5 alloys to fit tyres to - £240 +£500 tyres for 16", £300 +£600 for 17"s
The cheapest option is a set of 16"steel wheels and winter tyres, provided you are happy with the looks of steel wheels n trims !
The best site i've found to compare prices (including a winter wheel set option on the left hand menu) is
www.mytyres.co.uk