Hey what's up! I am also the owner of a 1989 240sx. You may not know this already, but your car probably has the single cam motor (unless previous owner swapped it to dual). What you should do to the car ALL depends on two things: what you want out of it and how much money you are willing to spend. You always get what you pay for. If you just want the car for handling and not so much speed and you don't have a lot of money, swap a dual cam in there and put coilovers on the car. If you want more power, the SR20DET as others stated is a great motor. It's what I have in my car. It's pretty tame (especially stock) for a turboed motor, but you will definitely also need suspension. Stock suspension is SHIT, especially on a 16-year-old car. A few things you could probably upgrade either way are electric fans, upgraded radiator, clutch, etc.

Is your 240 a coupe or a hatch? Just curious. I have a coupe and I <3 coupes.