No such thing as a 'boplt on' turbo. All turbo's bolt on. For starters do you know how a turbo works? If you dont then have a read...
Exhaust gasses go in the red side and spin the turbine. This spinnign motion is sent down a shaft to the compressor which sucks in air and sends it into the engine at high pressure.
More air + More fuel = bigger bang.
Bigger bang = More power
Problem is when you start stuffing air into your engine you increase stress on rods, pistons crank etc. I'm not exactly sure what the 1.8 Zetec engine can take (more of a diesel man me). Chances are you will either have to uprate all the inards of the engine or you can add a decompression plate onto the head. This decreases the compression within the engine lessening any stress on it. Also the turbo will need an oil feed to keep the sheft in between the exhuast and exhaust wheel lubricated. Again not sure how easy this is on the Zetecs. Then you also need to think about intercoolers. An intercooler is basically like another radiator that sits infront of your car. When air is compressed by the turbo it heats up, the intercooler reduces the air tempreture. Colder air = More oxygen = Bigger Bang = More Power!
Instead of an intercooler another option would be water meths injection which is cheaper but probably won't get you as much power as an intercooler.
And if you don't know how an engine works I wouldn't bother.
Then again I wouldn't be suprised if theres an off the shelf package avalible for the escort GTi somewhere, but it will probably be more than what the cars worth! I know theres lots a 2.0 Zetecs that have been turbo'd and supercharged. How about sticking one of them in the 'scort? Or even an ST170 lump?