My garden design fee.... is $600.00 about right for this job?

You will be working for a lot less than $27 an hour if you charge $600 for this job. Landscaping rates depend, in part, on where you live--in California, I'd charge $1,500-2,000 for a job of this size and complexity--but other parts of the country aren't as expensive as California. Still, $800-1,000 seems more like a reasonable bid for this much work.