I have just started a company, a C corporation. I want to start a pension plan by contributing the maximum possible amount into a money purchase plan and managing the assets (stocks) myself. Can I just open a brokerage account, call it XYZ Pension Fund, and do it?
Unfortunately, it's not that simple. Although your plan sounds sensible, you cannot just set up an account. Retirement plan assets must be held in a trust.
In addition, current tax laws have made money purchase plans obsolete. Several other types of plans might work for you. Which type is best depends on whether you want to contribute to your employees' retirement (some plans force you to) and how aggressively you want to contribute to your own retirement (the various types of plans have different annual limits).
A defined benefit plan is the most complicated and expensive type of plan to set up and maintain. You have to hire a pension planner and an actuary to set it up. But this type of plan offers the greatest tax benefits, so if that's what you're after, it may be worth it.
Other plans are easier to set up, and you can find a financial services company or brokerage firm to do it for you free or for a nominal charge. (Most brokerage firms have boilerplate plan documents, so you don't have to pay to have them drafted.)
For a detailed breakdown of all of the types of retirement plans, see Tax Savvy for Small Business, by Fred Daily (Nolo).