If you are planning on growing stuff after any SHTF, you should be growing now - I would say you should be growing at least 50% now of what you would be planning to grow post SHTF, it would be hard to more than double what you cultivate. You need to get your land in good heart and get your growing skills going now.
Think about the things that anyone growing decent amounts now buys in. I buy in a couple of sacks of pelleted chicken manure, couple of sacks of blood, fish and bone, couple of bags of dried blood, 1/4 - 1/2 ton of potting compost depending on what I need that year.
Sure, I could get away without these, but I would not be as successful. Post SHTF I would be composting ALL of my waste however so that would add something back.
As for seeds, if you are going to buy in bulk, rather than buying small packets, go to a company like this.
http://www.tozerseeds.com/
Some stuff from there you pay 1/6th of what you would on gardeners seed supply sites, garden centres and so on.