Hiya,
Firstly WELL DONE for asking questions and thinking about it BEFORE getting your pigs - so many folk just rush in with little or no forethought
I'd advise waiting til spring for a few reasons
1) It'll cost more to raise them to weight in the winter as most of their food be used just keeping warm.
2) You'll need a more substantial house for them over winter (especially if it's a winter like last year) Spring to Autumn raising you can get away with something made from straw bales or pallets.
3) Ease of tending them - it's no fun trailing out, in the dark, cold winter days and your fields would get more cut up and really muddy - pigs DO like mud, but only when it's warm. Cold, wet and mud makes for unhappy pigs (and can lead to arthritus)
4) You'll want to spend time with them, they are hilarious and great fun - it's more enjoyable doing it in better weather.
Obviously these are just my thoughts on it, and by all means it is possible to do it autumn to spring, it's just easier and a bit cheaper to start in the springtime.
HTH
Karen