Yes, you are disturbing/distributing weed seed if you bush-hog. After 10-12 years of neglect, however, it shouldn't make any difference in weed pressure, your ground will be full of seeds biding their time, waiting to erupt. Most weed seeds use sunlight as a trigger to germinate, they can lay dormant for 10+years until stirred up and exposed to light, then they begin to grow. My recommendation is to divide the 36 into six, 6 acre paddocks, bush-hog them all now, but fight and conquer each one individually. You will need 2-4D, fertilizer, and time. Using this plan of attack, you can do it right (at least my opinion of what that is) and bush-hog or burn, disk, harrow, reseed, fertilize and drag each sub-plot individually as funds allow. Once 2 paddocks are well established get a couple steers. I don't believe trying to salvage the old grass, whatever that is, is a good idea. You have no idea what it's nutritional value is, or how it will stand up to traffic or grazing. It's a safe bet it won't choke out weeds, look at the state of the field now. I don't see a need for a seed-drill, I've seeded many fields with a broadcast spreader (3pt hitch inverted cone) followed by a drag or harrow. I'd be waiting until spring 2009 to put any animals on that ground. I've never been on the same journey your asking about, but I'm an ex-orchardist and this is the approach I would take if I were to plan on establishing a new field of trees or tame ground for a blueberry patch. Good luck,
Ben