What is the significance of 3500 kg weight?

From threads on this forum I understand that if the motorhome is over 3500 kg then a medical is required to maintain a driving licence if the  driver is over 70 years of age. Can anybody be more specific about the exact requirements? Are there any road tax or MOT implications? Is this the total permissible weight or MIRO? What other restrictions might apply when touring in Europe as I only know about the French speed limits and not sure about Autoroute tolls. Should any of this affect a decision on buying a motorhome?