With only 7.5 acres, I don't think you would put much more than 25 hours per year on a 4020 unless you are moving a lot of snow, or grinding feed and hauling manure for livestock. Fuel costs will not matter much whether you run a gas or diesel. Transmission problems could cost $5,000 to repair, and will drop your resale value to below $3,000 as-is. I suspect the seller has found that out too. I think $2,000 to $3,000 is all I would offer on the 4020D with a bad transmission.
$3,500 to $4,500 should buy a 4020 gas in average to very good condition. Case 930 Comfort King diesel and Farmall 706 gas also sell in the $3,000 to $4,000 range. None of of those tractors are well loved, but any would work well for you at a very decent price.
$3,500 to $4,500 should buy a 4020 gas in average to very good condition. Case 930 Comfort King diesel and Farmall 706 gas also sell in the $3,000 to $4,000 range. None of of those tractors are well loved, but any would work well for you at a very decent price.