If your talking about breaking new ground you'll want a minimum 500cc atv. I'd also suggest one thats lower geared, has more torque or grunt. Some of the speedier geared atv's won't handle a heavy workload as well.

However if your food plots have a thick sod I'd suggest using a tractor, and plow to turn the soil over, let the sod rot a year first. If your plots are not of a thick sod, a atv with a plow disc setup will be fine. I rented a good disc set from Gander Mtn for a wknd to do ours.

Far as weed killing I usually do mine in early fall after brush mowing it,then spot it again in spring. I also use a bushog cutter behind my atv for that. You'll need something similar to mow your plots. But you can spring spray it also. I went the cheaper way, and just used hand sprayers mixed with roundup. I had someone sit on the back atv rack, and I drove slow as they sprayed it.

Other advice is if you need to lime, the lime needs a few months to work into the soil. We lime ours in the fall for spring planting.