Depending on how much land you have, could you fence off part of a field, or use a small field as a sacrifice area? I have Belted Galloways and Highlands and they live outside all year. If you feed hay over winter and move the hay rings about, then the puddled area will regrow grass in spring from the hay seeds that have dropped. When it's really wet and you can't get on to move the rings so it gets all boggy round them, I put some straw down so the cattle have somewhere dry to lie down. This rots down in spring and forms a nice compost. If a fairly large area ends up all rutted then I harrow it to level it out and form some sort of seed bed, and broadcast some grass seed and then roll it and it's usually all good and grassy again by hay time.
Buying in in Spring and selling in Autumn means you buy when prices are high and sell when they are low. A better plan might be to keep a few cows that calve in Spring so can take advantage of your spring and summer grass, then sell the calves in Autumn so you only have your stock cows to keep over winter. Also you are keeping a few animals for breeding that you get to know and form a relationship with. These are much easier to handle as they are used to you and you know them and you can get pleasure from your little herd instead of buying in a load of wild youngsters each year.
I would go with this idea. I live in a solid clay area, when we bought our small holding we could not understand why there were so many sheds, they obviously wintered the small dairy herd in them. Choose your least wet ground, and put down some large bale straw, you need about two bales to start, lay in flaps,like a carpet. Then put another bale out for them to eat in a round bale feeder. This creates an island out of the mud and somewhere dry for them to lie down.
I used to be on clay and in a flood plain and this worked really well with horses, and they are a lot harder on the ground than cows.I do not feed much hay, if they are native types they should do OK on straw, and the waste is less painful. Mine get a bit of concentrate daily, just so the get used to routine and a small bit of hay, more after Christmas when its really wet, enough so they eat it all so there is little waste. I would look for spring calving cows, so you are only wintering the cow, not a cow and calf.You then have a few months at grass to decide what you want to do with the calves, perhaps think about how you are going to get the cows back in calf.
My cows have been late summer calvers and it's a bit of a pain, so I'm going to move them over to spring. I have been selling them weaned at about five months and just keeping the odd one back for the freezer.