Hopefully - this is an interesting idea and one I have been thinking a bit about too. The problem I have is that I grew up on a large farm so am trying to scale that experience down by a factor 100 which is perhaps not the best way to go about it.
Anyway, this is how far I got in my thinking.
The first limiting factor is how much land I have (or will have). Stating the blindingly obvious, if I have 5 hectares of land it is easier to be self sufficient than with 0.5 hectares. More land means more food production capacity.
Choosing, to be 100% vegetarian rather than eating veg + meat would also make it much easier to be fully self sufficient. From what you are saying, you are planning to share a pig/cow but will you be growing the food to feed it?
The reason I ask is because my understanding is that it takes about 10 Kg of plant protein to produce 1 Kg of meat protein. The animal also needs carbohydrate on top of that to sustain it while it is growing of course. If I were just 100% vegetarian I could just eat the 10Kg of protein plus the carbohydrate myself rather than just 1 Kg of meat and hence be more self sufficient with less land. If I just bought a share of a cow/pig and let someone else feed it and they also buy in the feed grown elsewhere it would feel to some extent like cheating to me. I guess it depends how 'ethically sustainable' I/you want to be.
My feeling is that if I kept several nanny goats (for milk and fattening the kids), ducks and chickens might be more truely sustainable and more in keeping with small scale husbandry than a shared pig/cow. I have also been thinking of hosting bee hives in return for honey share. Not sure about the economics of honey or the likely yield though.
I agree with Fibonacci on the vegetable growing issues. The main question again for me is how much plant protein I can grow. Also, what about winter and should I be trying to grow fruit throughout May - October?
The energy issues are a bigger struggle.
Collecting power via wind is uncertain and weather dependent as is solar photovoltaic. Your idea of selling power to the grid when you have a surplus and taking it back when you have a deficit is a good one. The problem is that, as I understand it the price I would get for the surplus power is a lot less than I would pay when in deficit. Maybe I am wrong on that.
My alternative is a ground heat pump plus a simple solar hot water system to provide space heating and hot water. Cooking would be with a wood burning AGA/Range and that would add a bit of top up space heating and hot water. There is no way I could grow enough energy as wood though so I might need to collect windfall wood or buy in logs in exchange for say some hedge cutting work. I might then be able to get away with windpower and sell/buy from the grid just for low energy lighting.
I don't have a car so would rely on public transport, walk, bike and occassional taxi so would have to earn enough to pay for that by working locally or via internet.
I have not thought through the total CO2 offset as the calculation is so very complex. The default position is I could buy a few carbon credits.
Water, we could fit a grey water system for loo flushing and atering the land but without a very complex filtation and purification system I think mains water would still be necessary for drinking/bathing/cooking/washing.
Well thats how far I got. It would be very interesting to follow your project so reading your future posting on MN and links to your blog would be fascinating.
Good luck and enjoy the experience.