I wouldn't buy a 1.6 100ps focus, it's not a light car and the engine struggles sometimes, the later focus has a 120ps vct 1.6 if you do want a petrol that has a little more grunt
1.6 diesel is good in the 04 onwards, need oil changes every 6k to stop turbo issues and injector seals fail but there good for well into the 50mpg
Saab engines and ecu are very tuneable no need for aftermarket if you can retain the oem ecu
Powerwise, you can never have enough, it's only goes as fast as you press the pedal I have around 620bhp per ton through the rear wheels and have no issues just a large smile