Buy the tractor if he is selling it for a cheap price.

When you get it home, install a pair of new, correct spark plugs in the engine, as recommended by the engine manufacturer. That way, you know that you are putting plugs in that are the correct heat range and length.

Fire up the engine and let it warm up completely before shutting it off and changing the oil and oil filter...again with the products recommended by the engine manufacturer.

Running the engine with the choke on for an hour or so won't instantly destroy an engine but it does make the engine run excessively rich. An overly rich mixture will foul spark plugs and it will contaminate the engine oil. It will also wash the cylinder walls down and remove the lubricant that is supposed to be there to help control wear on the cylinders and piston. However, a mere hour of this condition isn't something I would concern myself with all that much.

With new plugs, the spitting and popping should go away and the oil/filter change is being done to get rid of any excess gasoline that may be affecting the ability of the motor oil to do its job.