A p&p'd head aids flow etc. The basics of gases in gases out and ultimately will aid power. Coupled with induction and exhaust mods it would be worthwhile if its inexpensive for you to do. That said, the ST170 engine is regarded as the best zetec ford produced and essentially already in a good state of tune. People have reported losses from changing the exhaust as it is tuned perfectly for the car.

If you've not already use a K&N panel filter in the standard airbox, then get a deflector from an auto focus to go in slam panel that directs air into the snorkel, remove the resonator box from the side of the airbox too for the induction noise.

You could decat it but i'd say you will need to custom map it to make the most of the mods.

An off the shelf obd plug and play remap works well by all accounts, dreamscience being regarded as better than bluefin.

Brakes, i've always liked ferodo, they can be noisy but i've always found they last well and perform as good. Don't be tempted by cheap pattern parts for brakes, i glazed the cheap pads and the discs with good pads still fade far too quickly.

I've found Falken tyres to perform well and aren't stupidly expensive. However.....these are heavy on tyres they wear the inside edge both front and rear, we usually have a pair every year on the front. There are better performing tyres but its a trade off against cost imho. The real pricy brands may perform better but you'll likely still be replacing them as quickly