Good little project but I think you also need to consider:
* Loom, the ST170 ECU and loom will be completely different to what is already there, what is your wiring like?

* Engine mount for the blacktop is different to that of the silvertop. What are you like at welding

* If using the 6speed or MTX75 you will need to make up some custom mounts and start adjusting the chassis rail (Careful here as you may affect strength)

* DriveShafts when using the 6 speed or MTX75 will need to be custom made as the length will be all wrong (few hundred quid)

* If you don't use the 6speed or MTX75 you will need to find the clutch and flywheel off a BC/IB5 Zetec box as the ST one is a DMF and is the wrong size for the BC box. ALso be aware that the BC\IB5 box is quite weak so if you drive it hard be prepared for a lot changing

* Exhaust is at the front of the engine rather than at the back so you will have to customise the exhaust to some degree. You may find inspiration from the MK3 RS1800 as that is the closest to what you are trying to achieve.

* Coolant pipes will need to be adjusted for the Fiesta

Considering that the ST170 motor averages around 155bhp and the cost of the above if you can't do much yourself, have you considered the 1.7 Puma engine with Racing Puma parts to bring it up to 155 to 160bhp? You won't have quite so much torque but the 1.7 revs better and it is lighter so won't feel so nose heavy as the car with the st170 engine.