Before getting into the 70k question, have you considered the gift tax, stamp duty, and capital gains tax for parent implications? Likely agriculture relief (reduce 90% of value of assets) will apply for gift tax and retirement relief will apply to parents.
Because it’s a transfer from parent to son, the 70k will not really come into it (Apart from reducing any potential gift tax). I am not sure I understand about writing off the 70k against stock and machinery?