First thing's first, deal with any rust and rust proof as much of the body as you can
Then deal with reliability - tidy up the wiring, make sure that all wires and cables are away from anywhwre that will become hot. Get a battery immobiliser, fuel filter if it doesn't have one, decent lights and so on. Then if you're wanting to increase power, you'll want to upgrade your brakes first, so steel braided brake hoses and whatever else you want to do with them. Then, once you've got a well rust protected, reliable and relatively safe car, you can start to have fun with engine modifications, cosmetics etc.
If you're a relatively new owner, look up the MoT history of the car and see what issues it's failed on and see if these still need dealing with.
Edited by WillCarter, 10 July 2018 - 09:28 PM.