You should probably write the plan on your own. Since it is your company, it behooves you to know what you want to do with it rather than having someone help you with that. If you're worried about formatting or making it look like a standard business plan, I'd concentrate more on content.
Here's a good blog post by Steve Blank on the subject:
We have done a range of these from both sides of the table and would suggest you find some guidance and samples online to kick you off but then do it yourself from there.
We have used business adviors, they sat with us over several 2-3 hour sessions, asked a lot of questions, and wrote down all our answers. It seemed like a good process, and probably was given we were just starting out and didn't know what we should cover ... we filled it all in, were very happy with our new plan ... and it then just sat on the shelf.
For about 5 years I rewrote the business plan about once a year. It is kind of my working scratch pad, once an area, like employing people or sales pipeline gets finished I lift it out and put it into a formal spot for everyone to refer to.
These days I pretty much have an internal / private blog which serves as the business plan, its good because everyone can see the goals and can comment on them as well. I then lift the final versions into our wiki.
It depends what you want the business plan for, how formal your organisation is going to be and if you are going to approach a bank for money.
If it is the approach a bank for money then I would consider it to get other to cover off all of the details in the cashflow.
If it isn't to go hunting money straight away, then I would look to write around 3-5 pages and do 1 or 2 sheets in Excel (Incoming and outgoing by month for 2 years). Anymore for the sake of writing isn't really productive ... if you get on a roll like I do then it can be a great way to work thorugh ideas.
It all depends on what extent you are looking for. For the various sub components there are lots of individuals who are willing to assist you with their various skillsets.
I strongly recommend writing your own business plan because only you truly know your business, your customers, and your product. The process of writing your own plan will force you to ask yourself all of the hard questions about your business that you need to know the answers to.
If you are worried about the format or if your plan is believable, look to get some help with your plan once you have put together an early draft. You can get a lot of free help from your local SBDC or SCORE office. Both of these organizations are funded in part by the SBA and are there to help you. Or, hire a consultant who will work with you and give you feedback. But, don't have the consultant write the plan for you.
At the end of the day, you (not your business plan consultant) will be presenting your plan to investors. It's critical that you know your plan backwards and forwards and that you understand all of the assumptions behind your sales and expense forecasts. If you outsource plan writing, you will never know your own business as well as you should.
A financial planner is the kind of person you want on your side when you want a business plan written. A good financial advisor will sit down and make sure they understand every single aspect of your business, and then help you lay that out in a nice little (or big) business plan.
You need to make sure before seeing a professional that you know what you want your company to be, demographics, sales estimates, target markets and expenses. You need to have the financials and inner workings sorted out, or you'll be spending thousands of dollars in time sorting it out with the planner.
I got a business plan recently written up for a couple of hundred of dollars which was pretty cheap. He was helpful and carefully explained everything to me in a way I could understand. As getting a business plan is the easy part, understanding it is the hard part.
It was a face-to-face meeting and honestly that should be the only way to go about getting a proper business plan drafted.
I use Microsoft Word. Works for me. Substance matters more than style.