1999 Ford Explorer Ford Explorer, please help

Im looking to purchase the car above and I was wonder is that mileage too much at the price of 3990 and would it be worth it to by. I have also been reading a lot of reviews for this particular make and model and alot of people end up having to either flush or replace their transmission so how much would that cost me if I had to do that for future reference and with this being a six cyclinder how much gas a week do you think this car will run me