My advice came from Rohloff in Germany. I emailed them to explain I was interested in a Rohloff hub bike without any kind of service history. I asked them whether it was worth buying. This is their answer in full:
"Unfortunately, I am unable to provide an answer as each unit will react differently over such a time period. It may be fine, it may not be. YOu see the SPEEDHUB is not 100% sealed as the main shell seals (and one within the hollow axle), have a dual purpose. These act as breatehrs to redfress air pressure imbalance. As air escapes, oil can be drawn out with it. As air enters the hub though, so can moisture be drawn in with it.
Worst case scenario - the hub will require a full revision at one of our Service Partners.
https://www.rohloff.de/en/company/worldwide
The cost of this is kept very transparent with us though. This service will have an MSRP of €299 (+/- for sales tax and currency fluctuation).
Mit freundlichen Grüßen / Best Regards
Stewart Stabik"
I then rang my nearest service centre:
Keep Pedalling (Service Partner)
23 Hilton Street
Manchester M1 1EL
Tel.: +44-(0)161-222-6015
They said the cost will be £300 to £500 depending on what they find.
The worst case scenario may not be the actual case but, being risk averse and not wanting to get stuck on a long haul trip, I would definitely opt for the full service.
The best of luck with the sale, this will probably be a brilliant bike for someone.