Welcome! You are now at the point I was about three years ago.
I'm going to start by tooting my own horn a little--I made up a buyer's guide for the first generation models and it's here:
I recommend looking it over to be sure you understand what equipment is and is not on the 2015.
I don't believe we have a lot of experience here with high-mileage examples. I've only got 18,000 on mine. Maybe somebody else can address that? So I can't quite say how everything holds up.
In general, I'm assuming you'll do the usual things--crawl under, listen to engine, check tires, etc. Inside, you'll want to push all the buttons (that will take a while) to verify that everything works. Two points that some owners have noted in threads here are chrome peeling off wheels and problems with the swiveling headlights, so you may want to zero in on those.
Three owners in about six years does concern me a little bit. The first one, who leased, clearly kept it until the lease was out, so that doesn't bother me. But I wonder why numbers two and three were not inclined to keep it long term? They must have had a reason.
As regards service, be aware that not all Kia dealers are certified for K900 service, so be sure you have one within reasonable distance. The car is somewhat eccentric, and not every independent garage may want to take one on.
The price seems OK, but with plenty of room to bargain. These cars do depreciate fast, and tend to sit on the lot for a good while. Both those are factors in your favor.
Let us know what you decide!