Just bought an 89 'Stang. Need Mod advice

[Mr 5 0]

Change the oil and filter immediately.

The, start with a basic tune-up.

New: spark plugs (Autolite #24 or 24), decent plug wires (Ford Racing wires are O.K. - don't go nuts with this), brass-terminal distributor cap, rotor, gas filter.
Buy a K&N panel filter if your '89 doesn't already have one in place; if it does, get a cleaning kit to clean and re-oil the filter.
Advance the timing to 14 degrees; be sure to use 92+ octane gas when you do this.

This simple tune-up (you do the labor) will cost you anywhere from $60. to $100.+ for parts and should get the '89 running right and give you a baseline to start from.