This is because you have said you have plans to find investors or sell your business. It is easier to take on investors if the business is a company. It may also be easier to sell all of the business.
Also, you have said your business is likely to involve large costs or debts. A company is a separate legal entity. You are not personally liable for company debts unless you have provided a personal guarantee. If as a director your actions are seen careless, you may be personally liable to the company for any losses.