Hi - I'm assuming that they are not covered by the Tacho rules so you need to think about the work they will be doing and how physical it is ie how tired are they likely to be at the end of the working day.  Also you haven't said exactly how far they will be driving.  So that leaves consideration of the working time regulations, in these situations I think that the driving time should be counted as working time and that should be where you start from.  Other than that I personally think that driving more than 2 hours and working for an 8hour day would be excessive.  Have you tried looking at your fleet insurance policy (or your insurer) for guidance?  Most policies / insurers provide loads of guidance on fleet safety and that might answer all your questions.

http://forum.iosh.co.uk/posts/t127507-Driving-Hours--vehicles-under-3-5T

http://forum.iosh.co.uk/posts/t126869-How-many-hours-can-you-drive-for

an exemption kicks in if daily driving is less than 4 hours in one day BUT in the event one day has more than 4 hours then the full week is subject to the rules.