Welcome to the forum.
I have a 2010 Enertia and the batteries are probably around 90% of what they were when new. It's a low-mileage bike. However, I run the charger about 1.5 times a month, year-long. I'd ask the owner to show you a start-up video showing the dash to make sure there are no fault codes displayed, at the very least.
You should know that Brammo has, basically, zero support for the bikes any more because they sold the electric motorcycle division to Polaris who has kind of abandoned it, so you will be on your own as far as any issues you may encounter. Still, depending on the price (and could you share that with us?), it might be a fun addition to your collection. I still love riding mine.