Courtesy of James K., Virtually Green, San Francisco, CA
If you're planning to bring in topsoil and do raised beds I suggest you consider putting down a layer of heavy duty woven weed cloth over the entire 20K sq. ft. site.
First do a thorough weed wacking of the existing weeds over the entire site. Rake up the wacked weeds. Scour the site looking for sharp objects, such as concrete chunks, glass, etc, that might pierce the cloth. Then lay down the weed cloth. Some people lay down a few inches of coarse sand before laying the weed cloth down to help prevent piercing it, especially where there's foot or wheel traffic.
Next frame up your raised beds on top of the layer of weed cloth.
For added insurance, lay down another layer of weed cloth on the bottom of your raised beds. Then put down the topsoil in your raised beds. I recommend at least 18" of topsoil if possible, and 24" is
better: deeper of course if you're going for root crops, like potatoes. It's wise to lay something down over the weed cloth visible between your raised beds, to protect the cloth from puncture from feet or wheels as well as from sunlight UV damage. A soft bark mulch is good.
Protected from sunlight UV and gardening punctures the weed cloth approach described above could last 20 years with little or no problems. You might have to do minor occasional repairs to pathways but that should be all.
Here's a couple websites that discuss weed cloth installation:
Keep in mind that not all weed cloths are not created equal. Many of the 3 oz or less weed cloths on the market are understrength for handling dock or ailanthus. You'd need a heavy duty cloth. Check out Dewitt Weed Barrier Pro. It's a multi-layer cloth with some great industrial strength specs.
It's from a company that's been around for a while. You'll no doubt enjoy hearing they have a 100% No Weeds guarantee on their weed cloth. You might buy a small roll of Dewitt Weed Barrier Pro and test it on a few spots around your property.