I have a Series II D-12. The machine you're describing is the ultimate in D-12's. About the only things it doesn't have that you would get on a new machine are 4WD and a differential lock. It should have fully live PTO and hydraulics and remote hydraulics as standard. Power steering and the multi-range tranny are hard to find. The first gear on the regular 4-spd is probably slow enough for tilling; with the multi-range it should be great. Most mechanical parts are available from AGCO-Allis or aftermarket sources. Sheet metal and the like generally has to be fabricated or salvaged.
The Allis-Chalmers have excellent traction for a 2WD machine. I had a Ford 600 with a loader and the traction was poor, even with a lot of ballast on the rear. The D-12 does better with less ballast.
For mowing you would probably be best with a 5 ft. rotary cutter, maybe 6 ft. if a finish mower.
The machine you describe should easily be worth $5000, maybe more. I paid less for mine, but it doesn't have the PS, multi-range tranny or factory 3-point hitch and wasn't running well at all when I bought it.