Is it necessary to incorporate a startup before you can get outside investment?

No investor in their right mind will put money into an "unformed" company. If you wish to remain unincorporated, you should seek out investors who aren't in their right mind. Depending on your company and its product, this may be necessary either way. Seriously, why do you think you should wait. In most jurisdictions, forming a corporation is a relatively inexpensive thing to do. I think it costs less than 20 pounds to form a private limited company. That being said, since you're getting investors, you should seek out the advice of competent counsel to help you with this.