The leon cupra can suffer all the similar 'common faults' as the other VAG cars of this era. (A3, MK4 GOLF, BORA, FABIA) so there is not one thing to look out for.
The best thing to check is that there is a comprehensive history of any service/replacement parts. Im not talking stamps in the book, but good old recipts.
Dont worry about the log book at all, but you should see that the oil + filters have been changed within decent intervals and that there are recipts for other bits that may have been replaced.
A car that is well looked after may have no faults for 10 years+, however a poorly maintained one may cause issues within 6 years of being new.
Many people on here moan and groan about their seat and its faults, but many people also buy one and never have any issues. Its all down to how its been looked after, and you want proof that it has before buying.