Judge Neil's Advice
I have never done professional landscaping, but from a general professional and common sense point of view I will be happy to give you some pointers.
First of all if you are in business you should really understand it fully or you are going to get yourself in trouble. If you cannot adequately scope the work you should probably turn it down. It sounds like you started out the service in a small scope within your expertise to make some extra cash. Once you get your feet under you and grow you can start to expand out into other areas.
Now if you still want to go after the work after I tried talking you out of it, you should address several issues...
1) To get an idea of the price to charge you should check out local companies and what they charge. You can use online yellow pages
www.yellowpages.com and
www.yellowbook.com to find a local vendor and pretend to be a client. Talk to them about the job and see what they are charging.
2) You need to get paid up front. Even if it goes against the industry norm. It sucks doing work and not getting paid. Even if he says he will pay right away, it sounds like you will have a subcontractor so you have to pay the sub anyway.
3) Create a detailed statement of work. Include specific details of EVERYTHING that will be done. Will you cut the tree down? What about the stump? Who gets rid of the rubbish? What is the time line?
4) Know your local laws. There may be rules about landscaping work such as having contract licenses. If you pull up a gas pipe with the tree roots who pays for it? Are you insured?
Sounds like a lot of risk here... My advice would be to pass at this point and may think about expanding later....