This depends on the budget
wether its a CMS build or ground-up development its still a large piece of work
You will not benefit from using a CMS other than provide the client with additional functionality for no additional cost.
It is a custom development. Thats what it is.i would love to use a CMS since his budget does not support total custom dev
Point him in the direction of his bank manager!
If he is serious then get him to fund the development seriously
if you do do it make sure you own the rights to it. You need full control over it and host yourself so you can claw back costs via update charges. Make sure you dont start any work without a functional / design spec which is signed off. Agree what you will do and never fall for the 'Oh i forgot to add this' malarkey