I've tried to use OpenBSD a couple of times, and it's been a bit of a struggle getting it installed (compared with most distros I've tried, Gentoo excluded!). It has a reputation for stability though; most often I hear of it used for file servers of one sort or another, or firewalls and the like.
I presume if buying new hardware to move the data is out of the question, then buying a whole new LTO2 tape drive for the backups is out of the question? You could get the whole 200GB on a single tape then, although tapes aren't cheap: £25 each (200GB) last time I checked.
For the transfer, couldn't your boss stretch to £50 for a 250GB external drive, and you use that for the transfer?