So if I took the airways routing, what altitude would you recommend and also which agencies will we need to speak to
You need to get VFR charts and plan the route - as per your PPL training.
The altitude will be according to airspace, etc. Have a look at that French site.
The route I posted is an IFR route for which you need an IR.
I'm also presuming that Marseille are GA friendly and there would be no issue landing there
The information I have does not mention PPR and they appear to have customs and avgas. But you should contact them to make sure.