If you need 4x4 to bush hog, disc, plant or most anything else you ain't doing it right. I run 2 wheel drive tractors all the time and if you need the 4x4 you are ruining something. I can't remember the last time I got a 2 wheel drive stuck, get the 4x4 stuck all the time, mostly pull it out with a 2 wheel. I have a massey 135 diesel (45hp), I paid $3000 for no loader. 2 things to remember with 2 wheel drive tractors, ballast, and brains. The rears are where the work should be done 2 or 4 wheel, that takes ballast. I have several thousand dollars in front seals and final drives to prove that. Time needed to get 2 wheel un-stuck is offset by time spent fixing 1/2 acre of ruts made by 4x4. Key here would be stop going deeper when you start sinking. Stop, and back out, its not rocket science, or as they say at NASA, come on guys this isn't drywall. As for hp for the disc, way too many variables, disc diameter, pitch, 1 gang or 2, sandy soil or clay. I don't think you are gonna run a 5 to 6' tiller seriously with 30 hp. I feel underpowered with a 5' behind our 45hp, of course thats an old heavy tiller tilling hard ground. I think you would be fine pulling the planter as long as you could pick it up.
Just my 2 cents, probabbly worth a penny.