I'm having allot of dificulty believing that the disk in the picture is made from solid stock. As just mentioned, one built out of tubing weighes 700 pounds, then one from solid stock would be 7,000 pounds. It just doesn't make any sense to me, that it's that heavy.
A super heavey duty one should still be under 1,000 pounds and well within the ability of a 40hp 4wd tractor. The added weight will help it dig down, but not so much that it will be all that big a deal to pull it. A bigger tractor would always be nice, but then so would a decent sized disk. 5 foot isn't gonna do anything for a fire break anyway. He'll need to cut it 20 feet wide to have any effect at the very least.
With the amount of acerage you have, and the long term need for what you are doing, you really need to get rid of that small disk and get something allot bigger. Disking one acre of clay with grass cut short on it took me two days to get really cut up nicely. After that, I could do it in half a day with my 35hp 4wd tractor.
Good luck,
Eddie