I got a late ST24, been reliable for me apart from remote central locking solenoids pack up! Early ST24s have dodgy black plastic water impeller, can be changed for metal one. Check boot for water/rust as they can leak water round rear lights.
Also, as said the IRMC can pack up due to heat, so 2nd bank of ports don't open and lose power. Also, check head gaskets as rear ones can be pricey to fix.
The early ST24s appear be based on Ghia X (97,98), so had sunroof + aircon etc, later ones (99,00) Ford got clever and found ways save money.
The very early ST24s didn't have RSAP kit fitted as standard.
The ST200s have engine changes UIM, LIM, throttle body, cams etc to give extra power, with about 195bhp average for the 200, and 169 bhp for ST24.

For price they are now alot of car for the money and great motorway cruiser.
I luv mine, but I'm biased.