Well, I'm no expert on turbocharging, but in November 1998 I did purchase my own Sunny turbo engine and boy did I learn a lot. I have read a good bit on the topic in both on the net and in the popular import car publications. I have a reasonable knowledge of the theories and the equipment involved in turbocharging a small 4 cylinder engine.
I would like to say first and foremost, turbocharging a naturally aspirated vehicle is not as simple as buying a kit or engine and bolting it up and driving. There is a lot more involved in this if you plan on having dependable transportation that will last longer than a few weeks.
Bolt on Mods:
First, the engine you want to turbocharge cannot have a high compression ratio. The traditional compression ratio for a four cylinder engine is 8.5 to 1. You can go a little higher with multi-point injection. Most non-turbo cars have 9 to 1 or higher compression ratios. You can get away with about 9 to 1 turboed but beyond that, and the boost you add with the turbocharger will cause pre-detonation and blow your engine. Also, the higher the compression ratio, the less boost you can pump into the engine.
So you'll be rebuilding the engine with custom low compression pistons to lower the compression ratio, and at the same time, you'll be strengthening the block to take the additional stress of the turbocharger.
Obviously the entire computer system in you car has to be changed over to the the system for the used engine. You will need to get the the ECU, the engine wiring harness, all of the engine sensors, all of the solenoids and actuators and additional electronics which control the turbo system. So ensure that a trained mechanic is with you when making the purchase.
Next up, the fuel system delivery will have to be upgraded. You'll need higher flow rate injectors, a higher pressure fuel pump and sub-pump, and a rising rate fuel pressure regulator. Without the additional fuel, the engine will run too lean under the boost of the turbo, and will detonate, and blow.
You have to convert the entire induction system, which includes intake manifold, exhaust manifold, air plenum, waste gate, intake piping, and inter-cooler.
Now we're done with the engine up grade or replacement, it's time to look at all of the other changes you have to make.