Hard to know what "fully use" means. Since the late 2013 Mac Pro was the "current" version until very recently, I have to assume Final Cut Pro X knows how to utilize the D700. I don't know about Beauty Box.
Repairs are through the normal Apple service network. Either you have a local Apple Authorized Service Provider or Apple Store, or you don't. My recollection for West Virginia is that they're few and far between, but you may be close to one of the few. If they need parts, they can usually receive them in a couple of days, and if they need to ship it out for service (not sure if it's necessary for that model), shipping times aren't going to vary much - there are regional service facilities serving the East Coast.
Will Apple eventually sunset? Of course they will. It's never a matter of whether, but always a matter of when. Since the Late 2013 was the current Mac Pro model until a couple of months ago, it'll be nearly five years before that model is deemed "vintage" (five years from last date of manufacture) or "obsolete" (seven years from last date of manufacture). That means about five years of parts availability and repairs performed by Apple. Compatibility with the latest version of macOS is more variable, but for less long-lived models it's commonly about seven years after the model's introduction. There are specifics regarding hardware configuration/chipset, etc. that make this a bit variable. Mac Pros tend to have a somewhat longer lifespan with macOS than other models. Since this model was sold as new until recently, I expect they'll get new versions of macOS for a fair number of years to come.
