How much icing do I put underneath a fondant cake? Do I let the icing crust first before I put on the fondant?

Depends how much you want, but you don't want to put it on too thick unless you're going to put it in the fridge to harden up before covering it. Traditionally you only put enough to cover the cake and let it stick to the fondant (more than a crumb coat but less than a regular amount for a cake.) but some people like to put it on thicker like a regular amount. But you really do have to have the icing pretty firm to be able to cover a cake like that without mooshing the fondant.

I aim for between 3mm-5mm of SMBC or ganache under the fondant.   I think that equates to about 3-4/16ths of an inch.  

Yes, if your are using a crusting buttercream let it crust over.  If you are using SMBC, IMBC or ganache then chill to set firm.