I wouldn't panic over the 300K number, but at some point you will have to decide on your "intentions" for this car. Are you planning on keeping it for the long term and doing "whatever it takes" to keep it in good condition, do you plan on driving it for some specific amount of miles, or are you planning on keeping it until it starts costing significant money to maintain/repair?

Once you decide what your plans are, you will know how much effort/money you are willing to spend. Realistically, this car is reaching the mileage where you will start seeing potentially expensive issues.