Oil Cooler
An oil cooler's purpose is to do one thing, and that is to cool the oil. What-the name didn't give it away? The reason you'd want to cool the oil is that oil is used for lubrication, and when oil gets really hot (as it does in a turbo motor), its lubricating properties deteriorate; because of this, parts of the motor can fail. Oil feeds into the cooler, the air passing over the heat sinks cools the oil down, and the oil is then returned to the motor. An oil cooler will for sure extend the life of a turbo and its parts.
Intercooler
An intercooler's job is to cool the pressurized air coming from the compressor side of the turbo before it gets to the throttle body. By cooling the air, you push the molecules closer together, making it denser. Cooler, denser air is better for combustion and causes a more violent explosion, which gives you more power. Another advantage an intercooler provides is a drop in air pressure with an increase in air density. This makes it possible for the turbo to make more power with less effort, lengthening the life of your turbo. Keep in mind that the biggest intercooler you can find is not always the best for the job. In the world of turbos, it's all about flow, so the design of the end tanks is very important.
Turbo Timer
Turbo timers are important for the life of your turbo; they allow the turbo to cool off after running under load. A turbo can get extremely hot (350F), and with some hard driving, it can get to a point where it will be glowing red (650F). A turbo timer will prevent the bearings in the turbo from "coking." What that means is that the turbo timer will run the motor of the car to get the turbo back to normal temps. Clean fresh oil can then make its way into the turbo and cool it down before the motor shuts off. By abruptly shutting down the car, the bearings and seals will cook in the hot oil because oil pressure was lost when the motor was turned off, and new oil is not pumped in. When this happens, it forms a film or varnish on the bearings, referred to as "coke." Most turbo timers now allow you to program how long to run the car and calculate how long it needs to run by measuring boost pressures and the amount of time the car was driven.
Cooler Plugs
The purpose of running colder plugs is to run the motor "colder." When you have forced induction, one of the byproducts of more power is more heat, and the hotter it gets, the more likely you are to predetonate the fuel and cause a lean condition. The general rule is that for every extra 75-100 hp, you should run one plug range cooler. You can also tell if you are using the right plug by running the car on a flat surface, bringing it back home, and pulling a plug. If the plug is black, you're running too rich or too cold of a plug, and if it's a white-gray color, it's too hot and you've created a lean condition; the plug should look light brown in the correct conditions. Plugs are often overlooked when modifying a car even though they can be the cause of severe damage to a motor, so make sure you have selected the right plug for your car.