Check the missing stamp is a missed service and not a failure to mark the book - I had to take mine back once to get the book stamped as it had been overlooked on a service. The Vauxhall Dealers have access to the service records.
This could have 3 years to go if the mileage is reasonable which would be great for peace of mind. I Imagine the garage would give you a 6month /1 year one with the car but its not the same.
The lifetime mileage will give a clue to the amount of running the ICE has done - the 15 plate may be a better bet if the out of warranty one has 100,000+ on it. A low Mileage out of warranty one with a full service history (12 plate or shortly 13 plate) might be a better bet. There are 14/64 plate ones about which have some warranty left that may be worth looking at.
Just noticed
@Hogzylla is selling his
Ampera at least with a forum one you can ask questions and get a straight reply.