Since you are looking to buy used and keep the cost down, just about anything decent will do. You can spend a lot of time fine tuning what you want, but with that much HP and 4x4, just about any 3pt disk will work. I have an angle iron one that I use twice a year. I bought it used on Craigslist for $300. After 8 years, I broke two discs planting my food plot for this years deer season and need to replace them before April when I plant my spring food plot.
If I had it to do all over again, I would prefer to have bought a bigger disk with a box frame, but I wonder if it would be worth the extra cost for something I use so little? I disk up half an acre. I've had four different food plots and my soil is hard clay. It takes many, many passes over it to get it to start to work, but then after awhile it happens, and the ground turns to powder. It's a wonderful feeling and something that I enjoy seeing happen.
For a food plot, you don't really need to disk down all that far, which means you don't need to have really large disks. Nothing that you will be planting will need more then an inch or two of soil over it, and most you can just broadcast and drag.
Ideally, hope to find an 8 foot, box frame, 3pt disk with lots of grease fittings. You have the power to handle it. I wouldn't go any smaller then six foot, you'll just wish you had a bigger one every time you use it. Since this is just for a food plot, all you want to do is turn over the ground. It's not farming, it's not gardening, it's just turning over the dirt.
Watch Craigslist, do a state wide search on Craigslist, check ebay, look on your local classifieds, go to local auctions and google what you are looking for in your area. I've found that when I need something, I often find it in a few months of online searching for a good price.
Eddie
Eddie