OK, I guess it's a bit late to say:
Did you check www.holidayautos.com?
They do great prices.
As for the car, it is front wheel drive, and reasonable size, although if all 5 of you have full ski gear, it will be a bit of a squeeze (and maybe not too fast either, depending on the engine)
Here is the UK version of the URL=http://buypower.vauxhall.co.uk/showroom/search/standardModel.jhtml?model=LS&brand=Astra&inSearch=
false&vehicleType=Car]Astra[/url], click on the Estate versions to find out more about the wagon range.
The roads should be OK, but make sure you have chains with the car just in case! The main roads to the resorts are generally clear, but if you're staying in a chalet off the main road, you may need them.
Hope this helps,