Do most buyers use a generic LOI template to save costs, or is it prudent to hire an attorney to draft something?

For the former, does anyone have a fairly standard LOI that I can use to fill in the blanks regarding purchase price, deal structure, etc.?

Also, is it customary for the due diligence period to start the day the LOI is executed or once all of the due diligence items have been delivered to the buyer?