That is a diesel Dexta not a Super Dexta built after 1960. Things to look for:
1. If it has been started using starting fluid run away unless you are prepared to do an engine overhaul.
2. Hydraulics can be a little sensitive due mainly to the Position Control pin wearing. This is a common problem on the US Ford range as well as the rear end is basically the same design. Not a major problem to fix and would not stop me buying. Some people panic when told they have to take the lift off to sort it. 3. Look at the four bolts holding the gear sticks on. If they are loose or missing it indicates the bottom of the gear sticks are worn and jump out of the selectors. Again no major problem as they can be built up with weld and shaped back to fit.
Remember you are looking at a 50 year old tractor so expect some problems. Here in England it would fetch around £1000.00 in that condition.
cdmn has told you about the serial number I would just add that all cast parts have a code on them something like A24B or 3A24. These codes tell the exact date the part was made and it will usually be within a few days of the tractor being assembled. Post a few of these and we can tell you exactly when it was built. The easy ones to find are on the gearbox side, the rear axle trumpet housings, the rear axle itself down under the footplate on the brake pedal side, the hydraulic lift near the front of the seat and on the clutch bell housing behind the front axle support bars.
Check the clutch linkage where it connects to the drop arm out of the clutch housing. If it has two holes in the clevis it is a "Live" clutch tractor, one hole standard transmission PTO, the Live clutch is more desirable when using the tractor on a grass topper or other PTO driven equipment.