This is a question that depends very much on what value you put on the first-owner warranty.
I wouldn't go over half the price of the bike new in that spec, at most to 60% if the bike has very low miles, i.e. is near new. If you wait for the sales, you can generally buy something near the spec you want for 75-80% of the new price. Two thirds is too close to the cost of patience.
This may seem harsh (especially to those who recently paid the full retail price) but the value of that first-owner warranty, effectively for your life, is very, very high. You should consider that, while a Rohloff rarely breaks, when it does it has to go to Bridgewater or Germany for repair; your local self-styled "bike mechanic" will just do further damage. You could be up for enormous carriage charges (both ways) unless you despoke the hub each time. (It's not the weight but the size of the wheel that gives the couriers grief, for which they charge you through the neck.) Plus unknown charges at Rohloff for labour and parts.
You can guess I value the guarantee, and the attitude that goes with it, and the service that results, including the guarantee's unspoken but widely known extensions, very highly indeed!
Andre Jute