This is definitely not my area of expertise but I have a few thoughts. If the concrete chunks are larger than baseballs or collectively they just cover a lot of ground I would take them away since they can be a barrier to roots even when covered with some soil. You definitely want to battle back your weeds before planting new seeds. You might also want to direct this question to the soil forum since those folks know a fair amount about bringing soil back to life. A good soil test from your county agent might be the way to start so you will know with what you are beginning. How many animals are you looking to retire to this land? 5 acres sounds great but for dozens of animals you would probably need to supplement their grazing with purchased feed especially in times of naturally low supply (winter, drought, etc.). For just a few animals you might get by as is. The type of animals also matters. For instance, my understanding is that goats are much less choosy than cattle. In fact, goats might be a way to do the first "mowing" of the land. Buffalo are more adapted to native plants than cows. A top dressing with compost and/or composted manure could almost never hurt.