I know what you are saying, and yes, on that specialized a singlespeed freewheel *can* be threaded on the hub, there is a zero chance it will have a shadow of a decent chainline. A bad chainline plus a wide-ish mtb tire, and most likely not the best chainstay clearance to get the chainring close enough inboard (before we even get to the crankset which looks to have non-replaceable riveted chainrings anyways)

[MENTION=451710]TreyWestgate[/MENTION] It looks like that singlespeed gravity is decent, it does not have the best parts on it, but they all seem suitable so you can just assemble it and ride, then upgrade parts as you like. Sizing will be up to you, but I have found that a larger wheel size like the 29 or 29+ ends up handling better for a 6'+ person, but it is preference.