yes I believe that you can take your car to any lexus dealer and ask them to do a stand alone hybrid system check. Mine is over 10 years old but it came with full lexus history, therefore I was not much worried; even though I purchased from a non lexus dealer.
Apologies I was not much clear about the hybrid warranty, the standard mechanical warranty for hybrid cars; as per lexus official website is 60k or 5 years, which ever come first; but you can get extended warranty from lexus directly that should take you to the 10th year, this is the jest I observed after reading on lexus forum and on internet. Please correct me if I am wrong. As you mentioned earlier that the car you are looking at is 2012 model, so if I am not wrong the hybrid system should still be under warranty; being less than 10 years old. anyway I am attaching the link from lexus uk website, which can give you some idea about warranties. the link is https://www.lexus.co.uk/owners/warranty/.
The other thing I can think off, you can ask the selling dealer to get it serviced at lexus, the link to their service pricing is as under, please note that the if your vehicle is hybrid, the hybrid system check is included in that service. the link is https://www.lexus.co.uk/owners/servicing-and-maintenance/.
Regarding the driving style, sorry I have no idea what it feels like to drive a Nissan Leaf; Sorry. But if we are talking about GS450h, no there is no difference in driving a hybrid compared to conventional car, other than it just moves effortlessly thanks to CVT gearbox. the only thing I can say about the gentle acceleration is, if you are super conscious about the fuel economy especially in the city, keep it under 30 and it will run mostly on the battery juice; but it will not last for ever so the engine has to kick in from time to time to top-up the battery.
GS450h is a performance hybrid saloon, so it is quite loaded up with battery cell, iirc the normal Prius holds 20 battery cells, however GS has 40 battery cells, so it will empty the batteries pretty quick.
When I am talking about the battery usage, please don't misunderstand when I say it will empty the battery means it will literary drain the hybrid battery. Every car is programmed to use battery in safe range, when it shows purplish bar (in a normal mode the bars are blue coloured), it means it is near to min ideal safe limit (say 80%); and the engine will kick in to recharge the battery; but as soon as it will be more than minimum safe limit and you are driving under 30, the hybrid system will kick in and turn off the engine without any judder.
There is mostly a top bar space, that remains empty unless you drive on along downhill stretch of road, which help to turn the battery bars in green, but again it is still programmed not to over charge the battery. Same when you leave the accelerator or apply the breaks the system start to recharge the hybrid battery. in this way it will keep the battery happy.
Regarding your experience about popping in a lexus dealer ship and having a feel about the new cars, I can totally understand where you are coming from. After getting my lexus, I had a chance to pop in my local lexus dealership, after looking at the cars there, I am of the view that lexus has totally changed the game, I went to a bmw dealership prior to going to lexus dealer ship, since then I am of the view that lexus has full potential to be a new market trend in coming future like once merc was, then it was bmw , then it was VAG group; and now mostly marques look the same and without character from distance. this is my personal view, every one has a right to disagree with it.
So if you will end up with a GS 450 hybrid , I believe that you will not regret it, though you may need some time to get used to new car. I have been through this phase and believe me I am not looking back rather I look forward to a more refined and punchy car.