If the letter of intent is accepted, then the due diligence period will begin. It will continue until the time agreed upon by both parties in which, at the end of the term, a binding contract is constructed. Terms may change during this time if certain aspects of a property, previously not disclosed, are discovered.
We have a saying in our firm that smooth transactions make successful transitions. Basically, if two parties cannot agree on the elements of the LOI, they will probably have a hard time negotiating while dealing with the minutiae of the purchase agreement. Normally, once the key points of the deal are negotiated up front, the rest of the transaction tends to work much more smoothly.