40 acres ... you'll need good farm gear for sure. Attempting this with "ATV" drag behind farming implements would be a poor route to go. And probably not doable at all without stressing the ATV's.
As for weed control. If these are only grasses, Round-Up (glysophate) will do just fine. Generic 40% is available down this way for about $45/2.5 gallons. If there are woody vines, briars and/or saplings, add 2-4-D to the mix. Might want to consider using a surficant as well. This helps the herbicide to "stick" to the weeds.
If I were you, I'd bush hog this field down prior to applying herbicide and do this ASAP. Let it sit 7-10 days, then apply the herbicide. Glysophate works more effectively on new growth. If the glysophate can stay on the plants a few hours, the job is done. The 2-4-D needs a 2-3-4 day period to really burn down the woody plants. The hotter the air temperature better.
Once the field is "dead", I'd disc or power till it up really good. I prefer the power till because it leaves a smooth seed bed. This will chop in the organic matter of the dead grasses and stems. But it will also "plant" any surviving weed seeds. So expect some weed grow back. At this point you have a choice .... plant your food plot now or hit it again with glysophate, and plant later.
My suggestion as far as planting is with a "culti-planter" type machine. When set up correctly, this machine can drop the seeds correctly spaceing ... plant a at the optimum depth ... and cover and pack the soil around the seed. We have done this with our fall plots with superb results.
For sure, as already mentioned, if you want a great food plot you should have a soil test made. And correct the soil according to the results. This will be the best $$$ that you spend.
As for post planting weed control. Either cultivate the crop ... or let the weeds go ... or plant Round-Up Ready varities, and control the weeds with glysophate application.
You might want to consider this fall planting a 40-50 yard wide strip of fall food plot crop along the edges of this field ... especially if the field borders a wood lot.