Hi! I'm making this wedding cake for a friend and I'm just wondering how you guys would ice the cake? Would you use fondant or swiss buttercream. Also would you add white to it? 

And I can't find a leaf cutter that shape any ideas on what I can use?

Also she is wanting cheese cake and a mouse cake. Would you guys just make a basic cake and then offer a slab cake to be cut? 

Any advice would be awesome!! 

WOW stunning cake, to have the cake that perfect and white it looks like fondant to me.  I'm in Aus so to get the edges that sharp it would be ganache under fondant to achieve that look.  The leaves are so perfect it is definitely a cutter that has been used.  Unless they are all as perfect as that the look won't work.

These football cutters from Ateco could possibly work for the leaf design...https://www.amazon.com/Ateco-5255-Football-Graduated-Stainless/dp/B000KEWP9U

that could be done in buttercream with the leaves in dried fondant 

that is so pretty

This is stunning!  As -K8mempis said, this could be buttercream with fondant accents, or you could do fondant, if you prefer.  It is pretty white.  You would probably need to add at least a small amount of white coloring.  If you are unable to find a cutter that suits you, make a templet from a light weight cardboard (like a cereal box, haha) and cut the leaf shapes with a sharp knife.  The leaves appear to  have had a vein put down the middle, which could be done with a dresden tool or a small ball tool, and slightly creased.

My very first thought is to send a message to the original designer/decorator. There watermark is on the photo. I would respectfully ask there permission to replicate the cake. Then ask what they used to achieve the look. Right from the source.