Check the under carpet in boot and wheel well for water, as rear lights can leak. If it an earier ST24 97/98 car may have black plastic water pump impellor which can break up, ask if changed to metal impellor.
When test driving you should between 3-4k revs should feel it pickup, this is hopefully secondaries opening, if not the IRMC box could be fried. Also the secondaries like a clean/decoke every so often as they get quite dirty.
As said check all the central locking on doors works, motors can pack up, about £80 from Ford, but about £10-£15 from a scrappy.
Suspension springs can snap, not always obvious, mine wasn't luckily stayed within strut.

May sound alot, but even later ones like mine are 10+ years old now so had a few problems over the years, but great cars.