You don't have to buy one. Just go to your local VZW and they will give you one. They may give you a hard time if your iphone isn't seen as a VZW one but otherwise it should be fine.
You may need a nano SIM that is already activated on your account since that phones IMEI number is not going to be in Verizon's database. The few that have got this to work got a nano SIM working in another Verizon LTE iphone on his or her account. Then take that SIM out of the working phone and put it in the iPhone6. Report back as this is not a sure thing.
You are correct. That was why I had to take my ipad mini to AT&T it would not pass the emeid check on Verizon's web site.
And the store would not give a sim even when I mentioned it would work per Apple and that it can be used as prepaid.