I have a 990 John Deere with front end assist,love it to. I use mine for food plots and the big garden that I plant every spring. Mine has a loader on and thats great for pushing debrie around the food plots, plus the weight that it adds. I use a 5 tine chisel plow to put my plots in, the ground is real rocky so a disk wont work well. I bought it new in 05, one of the best investments that I have ever made. Its small enough to haul on my on my 16ft trailer and pull with my F150. Its a 40 horse and really I wouldnt think that you would want anything smaller to pull a 4 row planter. 1sagittarius said it right the bigger ones are cheaper, but they are harder to haul around and i wanted one that I could haul. Take a look at Tractor house.com there are all kinds of tractors on there for sell.
Good luck
Greg