it's a strange proposition to consider setting up a bricks and mortar office based purely on that.
You mention using various API's to pull the listings in, but also mention setting your own commission rates. Are you sure this is actually possible when using the 3rd party data tools.
Surely the Zoopla API will be credited with any lead generation that comes from your website?
Sorry if im way off the mark here, a lot of what you've typed doesn't really add up.