Mike, no program will help you design. If you lack design ideas, "when in doubt, sub it out"! A good designer is someone that is educated in horticulture, drainage, hardscaping, construction, landscape installation and good environmental practices, and is able to put these together with your clients needs into an artistic vision. If you lack these qualifications, spend your money becoming better educated or better yet, pay someone that is.
I believe Pro Landscape is one of the better "design" programs and the price is reasonable. The inexpensive programs are designed for homeowners and that is where they should stay. They do not look or convey a very professional feel. If you want something to play around with, go with one of those, it'll be worth the small expense to learn. My best suggestion would be to seek out a local designer to work with, if the type of installs you are or will be doing warrant a landscape plan or 3D imaging. Design is a another aspect to your business as is hardscaping or irrigation. You need special tools, time and skills to do it well and profitably. Good designs should be paid for by the client and will definitely be worth the time and money they invest. I have seen some internet designs and am not at all impressed. You typically get cookie cutter or very basic designs with little input from your clients. A good design involves your clients thoughts, ideas and needs along with the site conditions, talent and knowledge of the designer and your ability to install the plan.
Don't just buy this software and let it sit on the shelf, it's way to expensive for that. Along with the software, you need a computer that can run it, maybe even a laptop and a large format printer to print the designs you create. Most importantly you need the time to learn and master the software along with the talent and ability to design.