Whether to use car batteries , or not, is up to everyone to decide for themselves.
The advice quoted is most likely advice given to the seller by Adam Harper. I would never say it is wrong, only that I have been using a mixture of car and leisure batteries for many years without any of the problems mentioned. Most of the C5's on the London-Brighton run in 2007 used freshly charged car batteries and they had no problems.
Many others on this forum continue to use car/leisure batteries and I've yet to hear of anyone who has encountered the problems described.
So whilst I accept the possibility that damage may occur , I have found that it is unlikely so personally I will continue to use car/leisure batteries.
There are a few other threads on this subject ;
http://c5alive.co.uk/forum/index.php?topic=95.0http://c5alive.co.uk/forum/index.php?topic=219.0C5Martin also advises car batteries are fine
http://www.c5martin.co.uk/c5_help.htmThe short answer is that I'm happy with car batteries