I would go with a minimum of 40HP tractor but for 20 acres 50 or 60 HP would be preferred. Before I bought anything else I would make sure I had a front end loader bucket for the tractor. I've been doing food plots for 30 years now so I have accumulated quite a bit of equipment. If I had it to do over again, I would have bought a field cultivator (vibra shank) before I ever bought a disc. I have a disc, of course, but I also have a 12' cultivator as well as a 6' - 3 pt cultivator and they can turn over dirt much more seriously than my disc.
I just bought a new Land Pride 606NT, no-till drill this year so I don't see using my cultivators or disc much at all in the future except for new plots I may open up as well as travel corridors and snake trails.. I have already sold my conventional grain drill and will be selling my John Deere 7000, 4-row planter soon. As previously mentioned, the planters do a far superior job of planting corn than a no-till drill does, but you can get OK results with the drill for deer food.
I am currently planting 16 acres and have finally picked out most of the rocks and stumps so I hope to progress into the No-Till arena for many reasons too numerous to mention if you don't already know. Consequently, after the tractor and no-till drill, the most valuable implements for me are a 3-Point sprayer, a 3 - Point cyclone spreader for fertilizers and a brush hog or rotary mower. That is not to say that I don't need my ATV sprayer and pull behind spreader, cultivator, disc, cultipacker, etc for travel corridors, snake trails and new plots so I probably won't be selling them any time soon.