Nothing too special. 2009 and up is an inch lower than previous models. 2018 saw fuel injection, carb'd models benefited from a jet kit as factory is pretty lean and could be harder to start in cool conditions.
Frame cracks is one of the things I always check for in a used dirt bike.
Seen enough of them to be concerned, is not necessarily a reflection on the bike, is more often a case of people not maintaining the bike correctly, but it happens enough to watch for it.
The KLX250 is nice little bike and I think would be great for what you want. I'm always confused when people say a bike is underpowered, or gutless, when they're not talking about racing. I race a lot and have a race bike (KTM 200xcw) for that. I had a 2007 KLX250 for dual sport duties and had lots of fun on it, so much so, that I wouldn't hesitate to get another one (well, maybe in 18 years when I turn 57).
For reference too, my first dual sport bike 7 years ago was a DRZ400. I would still take the KLX250 first.
Oh, and ignore anything Trials says. I've never heard of them having an issue with the frame cracking and I followed the thread over on ADVRider for a long time. With that said, I broke my subframe, but that was 100% my fault by having too much weight to far back.
The question asked was what to watch for when you go to look at used KLX dirt bikes.
He should be looking for signs of cracks in the area of the top rad mounts and the rear subframe specifically.
I had one in my garage not long ago and the entire rear hub and brake was trashed but that was neglect and abuse.
... and if you buy the fork seal parts from a third party vendor, half the parts they sell you might not fit well enough to use.