Go with one of the classics, because they're used for a reason. If you sound too stiff and formal, it comes off like a holiday card. If you reiterate your call to action, it can carry a whiff of desperation. Just get out gracefully and quickly once you've said your piece.
Understanding what a Letter of Intent is and when to use them is a bit confusing. But basically, this letter just means you are serious about continuing towards a purchase and that you want the other party to stop entertaining other offers. Our Letter of Intent template is suitable for all states.