I would go with Times New Roman or Arial, size 12. Studies have shown that those two font types are one of the easiest to read. But, I don't think it matters much, unless you pick some crazy font that's really hard to read. The content of your business plan is more important.
For font choice, you just want to choose a standard font such as Arial, Times, or Palatino. As long as you don't go with Comic Sans, you should be OK.
Images, charts & graphs, and other illustrations are very important components of your business plan. Remember, most people who read your plan will actually just skim it and only read the details of sections that they are most interested in. Images will help you communicate large amounts of information quickly for people that are quickly flipping through your plan.
For example, you should be sure to have charts that show your sales and expense forecasts as well as your profitability. A cash flow chart is also critical. You could also use flow charts to show how your product gets to market or what your customer acquisition funnel looks like.
Of course, including images of your product is always extremely helpful.
At the end of the day, though, don't spend a ton of time polishing the visual appearance of your plan. Focus on making sure the content is great and that it can be easily consumed. Save your time to work on a prototype of your product so you can demo that to investors. A great demo or prototype is worth much more than a beautiful business plan.
Agree with Zuly. The font type will not matter as much as the message you are carrying. Also, consider the tone of your business and your branding as that may affect the font choice.