Corporations and many other businesses can use this type of letter as a precursor to a formal agreement. In it they can list the salient points of their intentions and what they intend to include in formal contractual agreement if both parties can come to an amicable agreement. Thus it can be used as a fall back document should any questions arise during the final contract negotiations. A good example of this might be a letter sent to a company's investor explaining their intentions to buyout another company and what it is expected to do to the value of their stocks.
Here is an example of a good "Due Diligence Clause" you can use: "Seller to provide the following for buyers examination and approval in the next 30 days: 1) All leases, 2) All property management agreements, 3) All vendor contracts, 4) Current Rent Roll, 5) Property Income and Expense History for the last two years, 6) Year to date property income and expense history, 7) Last two years tax returns as they pertain to the property, 8) All units, buildings, grounds and mechanical systems. Buyer may void this contract at any time during this 30 day period if the information found does not meet buyers approval"