1. VVT - just under the oil cap, loud ticking noises... Potentially £700 fix (I've had done as a precaution)
2. Smoking turbo after extended idle... Another expensive fix (I never had this problem but bought a new turbo anyway)
3. Corroded alloys... Very common (I've had them)
4. Faulty air con... Corroded pipes (I've got this problem)
5. Inspect rear arch where meets bumper for rust/bubbling paint (I've had this but fixed it)
Thats the main things I would check out
Normally very reliable cars as long as looked after to be honest, good luck with the test drive
Kev's guide to buying a Mazda 6 MPS