In reply to Geras:
Haven't stayed in hostels in Inverness, but used both these places a few times:
sleeperzzz.com at Rogart - old train carriages, wee bit off route, but something different
Helmsdale hostel is on the A9, about 70 miles/1.5 hours North of Inverness, so would make catching the drive to catch the ferry fair bit shorter next morning.
Worth bearing in mind the A9 north of Inverness is much slower than the Perth-Inverness section, with few options to overtake if stuck behind slow moving traffic - we missed our ferry the first time we travelled up to Orkney!