Since AED's are medical devices, regulated by the FDA, I assume there are fairly stringent requirements on any authorized refurbs, so functionally, it should be as dependable as a new one and I would personally trust it. I would never purchase a used AED that hadn't been sent back to the manufacturer, for liability reasons on the job, if nothing else. Do these refurb units you found explain who did the refurb?

Someone with actual EMS experience could chime in here, but since you'll be out in the boonies, time will still be a factor since people whose hearts have been shocked back into normal rhythm often require more advanced care within a short time to stabilize their condition. An AED can resolve certain conditions that you can't deal with otherwise, but it's not the end-all-be-all for all cardiac emergencies.

I had my first AED training last year and it's not burdensome. It was part of my first aid + CPR class. Kind of interesting actually, since I had never seen what is inside the unit. Hooking up all the cables to your new flat screen TV is far more complicated than using an AED. I think all AED's have some sort of voice or text prompt to guide you through the process so the hardest part is knowing how to open and apply the electrodes quickly and properly and familiarizing you with the general process the AED goes through so you know what the prompts are referring to.

The legal angle is a crap shoot, if you ask me. But you're doing it with the best of intentions and if you get training, then it seems like you would be OK. Even if the person dies, it's unlikely that waiting an hour for EMS with no AED would have turned out any differently. But who knows? Anything you do on company time potentially opens them up to liability and if you aren't provided an AED by the company, maybe that can be twisted into some sort of maverick act by a lawyer. Good luck with this!