Most of my knowledge of the Zetec is centered around the Zx2, but the single best upgrade that you can do is get a pair of Crower cams, that and a better header (I've found OBX to be about the best of the aftermarkets.)
Also if the Focus airbox is anything like the Escorts/ZX2s the resonator is highly restrictive. My 99 S/R has the Roush CAI mounted to the standard airbox, and the later ones had an Iceman intake. Either of these would be a good upgrade from standard. The Spectre stuff is fairly close to the Roush in design and retails for about 1/3rd the price. Stay away from the K&N style filters as these motors lose power in most cases if the stock airbox isn't kept.
These motors don't seem to like OCIs over 5k miles. Between 3-5,000 miles on a conventional oil, it starts feeling kind of sluggish. On full synthetic, it starts getting kind of lethargic after about 6,000 miles. For this reason I always ran conventional or synthetic blend Motorcraft for 3,000 miles, until Castrol brought Magnatec out for 17 bucks a gallon, which I run for about 6k miles. 5w20 caused a lot of rattling and burned up too quickly, so I've always stayed at 5w30, sometimes 10w30 in the summer.
For oil filters I've always used anything with the unique anti-drainback valves made specifically for Zetecs that are supposed to help with startup chatter. Ford stopped making the FL2005 several years back but you can still get them fairly easy online. If you can't, try to find something that size that has a bypass valve setting of the 2005's 16+psi as opposed to the 10psi that you get from the 400s that it was replaced with. I get terrible startup noise running the 400s equivalents.
