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Is the car caught up on proper maintenance? If not, make sure you do that first. If you don't have a service history, assume it hasn't been maintained.
Good "first modifications" varies based on what you're looking to do with the car. Quality tires and a good alignment will drastically improve handling. A turboback exhaust and a tune will give a good bump in power. Driveline mounts/bushings will tighten things up and improve the overall "feel" of the car.