Welcome. My thoughts follow:
1. 2014s, some come with range extender. The one I had did not. Dealer may be able to test and give you current battery capacity. If not, then at full charge, no range extended should read about 86 miles range. With extra range extender charge, it might read 96 miles. This is after they reset the odometer to wipe out recent driving record.
2. Like all used vehicles, lower mileage generally better for wear and tear. Also, battery would not have had as many recharge cycles. I returned mine after 3 years with 9K miles total, very low, based on my limited daily drive. Average per year usage more like 7K (my guess).
3. You must test drive car. The motor should hum, not shreek. It should accelerate quickly. There should be no mechanical rattles. The electronics should be all working. Most of these cars were driven in warm climate in US, not the colder ones where electrics perform less well.
If the dealer offers an extended warranty that covers the motor and battery, worth buying it.
These are typical Mercedes build so should be able to perform well for you. But with new tech, I would only count on the car being good to you till 2024. But because so few were actually made compared to other models, parts availability will end sooner than with other Mercedes models.
Jeff in NYC
MB '17 B250e; MB '06 E500W; MB '81 380SL