thing to look out for on the st24/st200's are :

as said above , the water pumps break up as the fins are plastic ! make sure its replaced with a metal unit.

head gaskets go whenever they feel like it

exhaust manifold gaskets tend to develop a blow and are a bitch to change

other than that they are pretty good / strong cars

as for improving the power (st24) get a filter / exhaust / chip and a st220 (mk3) throttle body and have some fun !