2003 Hyundai Sonata Where to spend renewal money

We like our car. There doesn't seem to be anything wrong with it. I want to spend some money on making sure it continues to serve us well for a lot of years.
Where would you spend $300.00 per month for four months renewing the car to keep it for another 5 years.
I assume the first money should be spent on an inspection by a qualified mechanic.
The only unscheduled repairs that have been done on the car in ten years is AC compressor and one fuel injector controller.