Launched my own company in 2006. Spent 18 years in the corporate world with my background in marketing, purchasing and product management and I have left the computer and consumer products industry to fill a need in my community.
First of all are you going to be using the business plan to try to obtain outside funding or are you just doing it to confirm the viability of the business? It`s always a good excercise to write a business plan as a reality check on what you will need to accomplish to be profitable. Try to find a business plan (already completed by someone) for a similar business in an online search and use it as a guide to do yours. It may not be polished enough to present to investors, but at least you will see everything that your business will need to do to succeed. It may open your eyes quite a bit!
I think it`s a great idea. You will want to test and control who you give them to, but just about everybody would appreciate the gesture of being rewarded for being your customer. This will breed loyalty and create a unique opportunity for your clients to talk to others about it.
I use MailChimp.com and I highly recommend it. There are also a number of other email management companies out there like constant contact, etc. Google it and pick which one works best for you. Good luck.