There are plenty of sites out there for the Focus, I would start by going here to get some deals before buying new:
Since you have a ZX3, there are plenty of SVT appearance and performance parts for sale in the part out section
The SVT airbox and snorkel with a K&N drop-in is the best intake for your Zetec. Same for the header, none better than the SVT header.
Most of us go for an OEM+ look. You can do that with either Euro or SVT appearance parts. You will have to browse ebay.uk and ebay.de to find sellers who will ship euro parts from across the pond. Again, the best place to look for SVT parts ins in the Marketplace(link above)
Once you get into performance, you will have to decide to go boost or not. Either way, you will need a tune. One of the best Focus tuners out there is Tom (1turbofocus) check out the tuning forum for more info on that:
Check out Bat Inc (British American Transfer) for more euro parts:
Steve at Tousley Ford will give you the best prices on OEM parts, best way to contact him is here:
Nice Deals.uk has great deals on aftermarket Euro parts:
More euro aftermarket:
Best place for Rieger parts:
RS rep spolier:
Nice aftermarket grille:
Awesome phone mount:
New Edge Performance:
Headquarters for junkyard Focus parts:
I am sure I forgot some, but these are just some of the places that I have went for my Focus. Good luck and happy modding!