I bought mine when it was 2yrs old with about 13k mi on it. Since then there has been plenty of stuff that has required fixing or replacing, but it has never left me on the side of the road, and that's the kind of reliability that matters.
Like others have mentioned, be absolutely certain that you know the condition of the timing belt. These are interference engines, so if that breaks then it will cause very serious damage to the motor.
If the car was driven 'spiritedly', then you may have to replace things like the wheel bearings, tie rods, O2 sensors, and possibly the clutch, if it hasn't already been done.
Other performance / service notes would include:
-The rear brake pads on these cars are small...really small...really really freakin' small. expect to have to change them. At least it's an easy process.
-The Intake Manifold Runner Cable (IMRC) Control Module is prone to needing service or replacement due to cheap nickel plating on the contact points. there's a DIY repair tutorial on this site though. Easy enough to perform if you're careful.
-The OEM Clutch and Flywheel design was substandard and subsequently the design was changed post-production. If the clutch has not already been replaced, you will also have to buy a flywheel when you do replace it. Even still, this is an excellent opportunity for performance upgrades.
-The OEM motor mounts (3) were made from old grape jello that was left in the fridge for too long in the Ford engineering facility and feel like crap when you really get on it. poly replacements suggested.
-The OEM tune on the engine is notorious for creating 'surges' when maintaining speed and bogging a bit on take off. this is a combination of issues (largely F/A, VVT and IMRC related from what I can gather) that are easily taken care of with a tune. Most people on this site will suggest getting one from Tom from Focus-Power (1turbofocus on this forum) which will cost you about $400 ($300 for the flash hardware, + $100 for the tune itself), which comes with free updates as you modify your engine for as long as it remains NA.
Other than that, it's the usual service items that tend to be either regular maintenance, or car-specific wear and tear.