Hello Park and welcome to the SketchUcation forums.
Regarding navigation, if you have a 3-button mouse it is really easy. Scroll the middle mouse button to zoom in and out, press the middle mouse button and move the mouse to rotate around your model and hit the middle mouse button while holding the Shift key to pan. You'll get the hang of it pretty quickly. Took me awhile before I discovered this. It made navigation so much easier.
Also, if you go to View > Toolbars > Views and select top view you will be able to stay in a 2D view looking down from above. You can pan and zoom while in this view but make sure that if you rotate you go back to top view mode when you start modelling again. This prevents you from thinking you are modelling in 2D while you aren't and messing things up.
About your plant model generator, I don't think anything like that exists in SketchUp. However, search the 3D Warehouse as Bep said to look for 2D and 3D trees.
You could take a look at the SketchUp Ivy plug-in and try using that to make trees or bushes.
You could also check out Solo's Place for some 3D plant models. Also, try a search on these forums in the Component forum for free plants. I am working on a simple 2D plant component pack myself. I can give it to you when I am finished if you are interested.
Finally, if you cannot find what you need you could attempt making it yourself. Feel free to come to these forums if you have any questions.
Hope you find what you're looking for and that SketchUp will work for you.