The Visual Communication Guy shows us what a formal American business letter can look like. I find it very clear to understand.
I don’t use a colon after the greeting. I learnt to use a comma, but now sometimes just leave it out! I would never end a letter with ‘Sincerely’. I would use ‘Yours sincerely’. However, I very rarely write letters now. Most of my correspondence is via email. I presume yours is to.