It sounds like a great project. First off, A great design software is Raincad. It takes a while to learn so you may not be up to speed fast enough to use it with this project, but you may.
If you have a PDF or a plot plan of the property that will be helpful. You may even pay for a survey of the property and get an autocad dwg file to make it really easy to start the design in raincad.
If you want to email me or call me for some info you may.
I don't know about the GPS coordinates. A survey of the property will be the best. It would be great if your trees were on the survey or you could place them.
I am also in the houston area.
Good Luck.