who'd bother paying to "rebuild" it to OEM level, they'd always be upgrading in the process... so you'd have a better than OEM one cheaper than a "built" box, probably, as they have to disassemble and re-assemble a normal "built" one
of course it's the same ZF6 as any other turbo-6 variant of a ZF6, but wow, surprised they're so cheap... I would assume one that cheap would have been milkshaked haha
a ZF6 from an F6 should be $1k... didn't realise the price had dropped that far... admittedly I haven't been keeping an eye on the cost of them, but they used to be $5k+.
paying for it to be done would likely be prohibitively expensive...
have a read of this thread and it'll give you an idea
on how tricky it can be to move to a ZF platform:
https://www.fordxr6turbo.com/forum/topic/30284-ba-to-be-bfed/
all of those sorts of things in the aftermarket only really survive if they're excellent... just go with whatever you think is best value for your application.