One thing you are going to have to contend with is customization requests. I've helped white label several applications now and while the licensing/pricing may be fairly straight forward to figure out, figuring out if or when to allow and then how to handle customization requests will be key (especially if you ever plan to move beyond just one single client.) there are a lot of options you need to think through in terms of individualized custom requests (front-end related such as design or look/feel as well as back-end new feature development requests.) There are some instances where you may want to allow for customization on any scale but charge a hefty price and others where you should absolutely avoid it altogether. so in addition to the pricing question and contract portion be sure to think ahead of how you'll handle one-off customization requests so you aren't caught off guard when they inevitably start rolling in.

Best of luck!