17k isn't much: just look at what Gbillyg123
has for mileage.
I think you probably got a pretty good deal. The Niro is so new, that I doubt if any of the major rental companies are using it, so hard to guess what kind of a lease owned it before you. But most individuals who lease a car and have been through that experience at least once before have learned that they need to treat the car respectfully while they are leasing it, or they'll regret it when they turn it in. So hopefully your car was previously used by someone who is a serial leaser.
One thing you could do to address your doubts: look underneath. Rental companies look for obvious body damage, but never seem to look underneath. Not sure what sort of a checkup a lease gets when it's turned in. You might see some scuff marks on the panel under the front bumper, and that's kind of to be expected. If that's all you see, and if it's minor scuffs, I think you should feel pretty confident about your purchase, assuming you already checked more obvious areas of potential concern.
One thing you might want to do is run the tank low on fuel and then fill it with Chevron regular grade gasoline. It costs more, but it has cleaning agents in the fuel that can sometimes compensate for deposits left in the fuel system by less expensive brands of gasoline. I'm not aware of any reason why you should pay for premium instead of regular though - don't succumb to that old fallacy that it's worth paying extra for premium (higher octane) when your engine is not designed or tuned to either need or benefit from it. You might also ask the dealer that sold it to you to confirm that it was current on any service advisories from Kia.
It does seem strange that Kia wouldn't certify it, but I don't have a lot of experience with this and maybe rentals are disqualifying from the manufacturer's perspective, I don't know.
You probably got a really good deal, hope you love your new car. Welcome to the forum!