How easy would it be to add a factory turbo to my 1999 S70?

 

easy. Sell your car, buy a S70 T5. Adding the factory turbo would take swapping the engine (NAs run higher compression pistons than turbo engines), swapping the wiring harness (different sensors etc, different ECU) and then make sure there's no other part compatibiity issues (like the immobiliizer). Best approach would be to buy a parts car and then move as much of the components onto your existing chassis.