You can make the changes at a speed comfortable to you. Get recommendations from neighbors. Ask about your designer's education and credentials. Good signs include membership in the Maine Landscape and Nursery Association, and an associate degree in an area of study such as plant and soil sciences. A complete landscape plan may be several thousand.
Many landscape designers are also landscape contractors.
Make sure you understand what you're buying. A professionally installed landscape will likely be a lot more expensive than a plan. Though it's not the same, nurseries and garden centers often offer free landscape advice. Do bring in photos of your property and questions. Look for adult education courses in home landscape design.