@bix_the_dog Welcome to the forum. You are doing the right thing getting the collective wisdom of owners on here.
I bought my Tekna 30kw with 6-7 kw onboard charger 3 years ago last April and I absolutely love it.
Do the obvious things on here as others have written and when you do buy make sure you also buy a brand-new 12v battery as that is something that can give all sorts of mischievous errors and faults. If you start off with a new one it's one less future headache gone and you know it's condition. Buy a conditioning charger like the c-tek ones and use it once a month at least. This is true of all EV's they seem to treat the 12v system as an add-on and there is no way to top up charge it directly from the traction battery, and the automatic systems they employ just don't work too well.
I personally also highly recommend getting the 6-7kw onboard as you can then take better advantage of the cheap overnight tariffs. The 3kw will take much longer and run into your normal tariff. It's also better with the supermarket pod-points, it's hardly worth it with a 3kw.
Do the usual stuff as for any vehicle, rock the wheels for bearing and joint play, check under and along the sills for rust, inside the door pillars and particularly the rear hatch wells, if someone has taken the trouble to keep them clean and free of detritus, then chances are they will have looked after the rest of the vehicle. If in particular the hatch door wells have lots of leaves and pine needles, then it's not been well looked after. Check the rear brake brakes as stated in another comment here after driving a bit to see if they are hot. Look at the tyres to see if they are feathering on the edges for tracking errors or more serious suspension issues. The Tekna uses 17" rims (expensive tyres), the Acenta 15" which use much cheaper, have a far bigger range of tyres and are about 1/2 the price.
If you are a gadget man like me you won't care about the cost of tyres, you just want the all-round cameras bird's eye view, heated steering wheel etc. all the Tekna toys.....
Be aware the Leaf has an unduly massive "A" pillar and it obstructs your view of the oncoming traffic on right-hand bends, I have nearly come a cropper several times because of this. Get used to pivoting your body to look around this hazard. Frankly I'm amazed it ever passed safety inspections when first manufactured, it really is that big and I personally think it's downright dangerous.
Be aware Nissan have penny-pinched in many areas with the Leaf. For a £34,000 car, it has a pathetic boot light, no 12v outlet in the boot and no puddle lights, all should be standard on a car like this. Also the 2016 Leaf has LED dipped beam, everything else uses incandescent even the main beam. Absolutely stupid, my old 1981 Astra had better headlights.
Good luck with your search, hope you get a good 'un.
Cheers Tony.