I owned a 2011 diesel which I passed on to my partner and that has now racked up 200,000 miles and nothing has gone wrong with the engine, transmission, exhaust system. I now have a 2015 CRDi and I think the engine is the same. I recall some magazine rating the GD i30 as best in class for reliability. I can't comment on how good the automatic transmission is.
Batteries don't seem to last long, I had to replace mine at about 3 1/2 years old. I think the rear calipers can be troublesome with the handbrake mechanism seizing up. If there's no service history, you won't know how often the oil has been changed. Although mine is a 2-year, 20,000 mile service interval, I do an intermediate oil change every 10,000 miles. Failure to change the oil or using the wrong grade can lead to premature cam-chain wear.
Overall, I think reliability is tip-top, and you have the added benefit of a forum like this one for help and advice.