My $0.02: Get the stock spring, drive and enjoy the car. That way, you know you won't be stressing the engine too much, and also get to know the car. Do a Porsche Club Drivers Education or two, and see if it's your thing. A little more time with the car will be the best way to plan out future mods if any. Any well-instructed track time in a 930 can do wonders for helping you keep the car on the road during spirited drives. These cars aren't known for being kind to driver errors.
An intercooler upgrade is money well spent, and so is a newer turbo, suspension parts, brakes, etc etc...
Beautiful car, by the way.
Garen (87 930 track rat)