Removing graphics is a PITA at the best.
Once you have gently heated the vinyl off I usually wash the whole thing down with stamdard thinners.
This is a messy and time consuming job.
Remember this is why you have been offered this job.
If it was myself I would of turned the job down or gave the customer a huge quote for it.
Either that or tell them you can't quote the job and it has to be charged by how long it takes.
Good Luck
SC