Just what is included in that $865 for OW certs? As others have said, that sounds extremely high. At the LDS through which my wife and I did our OW, it is $310 pp, which includes the material, classes, gear rental, and admission into the dive park where the pool/OW training is done.
I would definitely check out some other dive shop/instructors and compare pricing. There has to be a reputable LDS and/or instructor that won’t see you as a sucker coming thru the door. And, yes, as others have said, don’t buy your equipment until after you have gotten your certs.
As for tanks, I suppose it depends on your circumstances. Our LDS is over 30 minutes away. It’s inconvenient for us to go to the dive shop to rent tanks. owning is much better for us, so we each have 2 tanks that we can load in the car and go diving whenever we want. As for fills, the dive park I mentioned above is only 4 miles from us and they do air fills. They also rent tanks, but only for use on premises. So renting there to take somewhere else is out of the question.