At the very least the first thing I'd suggest is to to as much basic maintenance as possible. Belts, hoses, gaskets, plugs wires cap rotor, suspension bushings and ball joints, etc. If possible also t-stat water pump things like that. Helps keep it alive. Beyond that I feel car modding is a very personal experience. However, there are plenty of build threads on here ranging from oem DD's to fully built race beasts. Plenty to pick from. Insofar as what I'd do, feel free to check out my build page if you'd like. Short version is, I'd focus on suspension and handling, make it ride nice. Then work on whatever you feel is lacking
