I purchased a high mileage '13 just over a year ago (93K) and have been extremely pleased. Fortunately the seller used the same dealership since new and I was able to get a complete service history printout from the dealership she used. Aside from general maintenance I did just after purchase including oil/filter, air filter and spark plugs, I've had to do nothing to the car. Being that you're looking at a '14 which is potentially 4 years old (build date), the only possible maintenance you may be looking at aside from and oil change is an upcoming coolant flush at 5 years/100K. You might also consider having the transmission fluid changed as although it's normally done around 100K, it's also listed as needing done around 45K under severe usage and without a record of how the previous owner drove it (if buying from a dealership) it's usually better to be safe than sorry. As for the test drive, definitely check the engine operation. This can be done either with a depleted battery or by placing it into hold mode. If the battery is less than about 1/2 charge, mountain mode will run to build the battery charge to 1/2 on the battery meter. This will give you a chance to hear how the engine operates. It shouldn't make any abnormal sounds and vibration should be minimal. In fact, only upon higher demand in charge sustain mode (battery depleted), its rare that I can hear my engine above a "normal" radio volume (12-15 on the volume dial). Also make sure the car has the included 120V EVSE charge cord. It's normally stored in the trunk. Verify it works, otherwise negotiate for a replacement or deduct cost from the price. Don't just plug it into a wall, but also plug into the car to verify the car does indicate it is charging.
I would also definitely push, twist and pull on any and every swtich/button you can find to make sure everything works. This includes the window switches on every door, not just the driver control panel. Also make sure all the quick access buttons on the each of the outer door handles locks/unlocks the car.
If everything checks out you should use your best judgement but I personally wouldn't hesitate if the car comes back with a clean bill of health. As I've said, I've had mine for almost a year and a half and I have taken several long drives in it, mostly 2-300 mile drives in a day and not a hiccup to be found.
A good indicator of engine mileage is to check the lifetime MPG of the car. It's found by pressing the button on the center console that looks like a leaf. Higher MPG indicates more EV and fewer gas miles. Mine had a lifetime rating of about 38 indicating mostly gas miles which onstar verified that out of my 93K miles, ~70K were gas miles, which is why I chose to change the spark plugs since those are due at 100K, which I felt 70K meant they were getting close anyway.