The first valve check should be done at 9k miles. Mine were within spec at that mileage, but many people need to adjust one or two valves. Also, that's when the plugs should be replaced according to the manual. When you go shopping, ask for maintenance records, whether it is a book, receipts or both. I'd prefer a used motorcycle from someone who documented maintenance to one from a dealer with no records or a person who supposedly maintained the motorcycle but never documented it.
Look for expanding fuel tanks. There should be a gap of about 1/4" between the forward bolt mount and the top steering clamp. The tanks expand due to Ethanol incompatibility. KTM will replace them under goodwill at least once, but if it hasn't been done and KTM decides not to goodwill the bike you buy because you aren't the original owner, yowl could end up with a $1500 bill. I had my tank coated internally so it wouldn't expand again, and so far so good. I wouldn't buy one unless this has been addressed. If the owner plays dumb, move along to the next motorcycle.
As Hotbrakes said, water pump seals go bad sometimes, but it isn't a tough fix as long as no one rode the bike hard or put a lot of miles on it with the contaminated oil. The sight tube on my bike split and started leaking when it was about a year old, but the replacement has never given me any trouble and I replaced it about 4 years ago.
Look closely at the gauge cluster for water leaks, holes in the mode/set buttons or the tach needle scraping on the instrument face. My tach had to be warrantied at 700 miles because the needle was scraping the face and there were little plastic shavings in the cluster.
Also ask to look behind the battery fairing or chin spoiler. A lot of crap accumulates back there and it takes work to keep clean. If it still looks ok, the owner probably was good about cleaning and maintaining it. When you change the oil, if you clean the sump screen on the left side, oil will drip into this fairing if you don't find a way to deflect it, and it will accumulate around the battery and make a mess. Not a deal breaker, but a good indicator of the owner's attention to detail or concern.