Consider hiring somebody from the outside if you have the budget for it. It’s not a bad thing that you don’t want to do it yourself, your time might be better applied to other business functions. Amateur business plan writing is about as good an idea as amateur surgery, cheap dentistry, and discount sushi.
When you do hire someone, look for a company that will be with you throughout the long haul. Your business plan will change with your success and you will need a business planning company that knows your history, shares your experiences and offers expert guidance concerning your market niche and style.
Business plans can become outdated quickly as your company grows. If you don’t have one you can run your business by, you don’t have one at all.