Good luck with the ST24 - like I said make sure there is a noticeable increase in power after 3500 rpm and that it pulls hard all the way to the red line. If there isn't, then the secondaries are not opening up - i.e. it is not running on all the valves.
This is caused by the IMRC (Inlet Manifold Runner Control) which sits on top of the engine and is affected by heat soak which fries the circuit board. A new one is around £50 - £100. If it works, then relocate it to underneath the battery tray ASAP!
The wiring that goes to the IMRC can also suffer from heat soak - this was sorted on the US cars under recall, but for some reason not over here. If that fails, then you need to take the inlet manifold off and clean the secondaries out yourself as they can become gummed up with carbon deposits.
Besides that, they are very reliable engines. The water pump has a plastic impeller which can fail and cause overheating, killing the head gasket.
It is a great engine, I sold mine to a mate who then sold it to another mate who both ragged f
k out of the poor thing constantly when it was cold, but 30k miles later it is still running strong.
They rust, so make sure to check under the boot carpet and the arches! Also a clutch change is a right PITA on them (as it is on all Mondeos) so if it is slipping be prepared for a big bill.
Wonderful cars, I would happily have another. Just be careful as they are now at the point where there are a few dogs out there!