I would sooner change gear than gearboxes
Expensive things them Auto boxes and they take the edge off the MPG generally 5-10 mpg which is likely to leave you where you started.
Looks a nice car on the pictures, 3 and a half grands worth? Im not so sure.Once that mileage goes up the price is going to drop like a brick.If it was a long term thing then resale value isnt an issue and it may well pay off.
Nobody can make the decision for you,but if you went for it,id be low balling him with an offer and work back up he wont have a queue of buyers.While we like to look at adverts and like to believe everything is right,it wont be so leave some left for the un foreseen.
The engines themself are a good engine pretty bullet proof as far as im aware.If its right now,theres no reason it wont stay that way for another 150,00.
Anything has to be better than a seat
Good luck,keep us up dated............