Hmmm bummer you didn't get any earlier replies?
My only advice would HAVE been - as with any car, check the servicing records. Also look the car over to see if it's been treated badly & yet tidied up for sale (harder to do with a dealer but you can always check to see if well cared for - hard to clean areas etc, is the engine area clean or a mess). But there's loads of generic advice on how to buy a car & look for issues.
So on an i30 of that PD series 1 type era - I'd avoid a DCT unless all drivers KNOW how to use it properly, it's not to be driven as you do an auto - especially in metro traffic.
And if you do live in a metro area - strongly consider if a diesel is a wise choice, as you will likely have issues with the DPF - amazing how many metro soccer mums you see in late model diesel vehicles and I'm sure they sniff a 100km+ zone for 30mins+ very rarely.
Otherwise let us know how you ended up faring?