A lot of traditionalist still like hand drawn plans for landscape design. I would suggest using any software you are comfortable with or can get easily. The key is going to be in creating an image library yourself. Create a set of blocks you can cut and paste in, pre colored, and textured so all you will have to do is paste them in and people will say, Oh, those are maples, those are tulips, that is liriope. If you do this (and I mean really spend a lot of time setting up all of your images) you will save a LOT of time on the back end and any software package can create awesome plans. AutoCAD Illustrator Photoshop (Although large images will get bulky very quickly).