So, you ask for advice. A number of people here who wish you only the best of luck advise that you do this in person, not by email. But you decide to do this by email anyway. OK…Now:
Scenario 1) You got lucky and the Cyber Gods ate your email. If so, offer many sacrifices at their shrine.
Then go see her. You’ve known her for three years, so waiting a few days or a week before you discuss this with her shouldn’t be that big a deal.
I read somewhere that in human communication, 20% of the message is in actual words and the other 80% is in voice tones, body language and facial expressions.
The problem with email and the like is that you can’t see facial expressions, can’t read body language and don’t hear tones of voice. You have no opportunity to respond to her reactions, and no chance to explain anything. You’ve limited yourself to 20% of your capability.
If you really want her for an SO, go see her. If you really don’t give a shit, send another email.
Scenario 2) You didn’t get lucky, your email went through and she has read it. In that case, if she were interested in your proposal she would have responded within, say, 20 minutes. There again, if she’s not interested in taking this to the next level it’s much easier for her to just do nothing, since you emailed her. Had you been there, face to face, you’d have had your answer before you left the premises.
In the OP you wrote: “I´d be hearthborke if this letter backfires and I end up loosing such a good friend,”
Why? A friendship such as this is going to be blown as soon as one of you starts dating someone else anyway. You might as well go for the brass ring and blow the deal while trying to serve your own best interests.
Oh, well. Good luck again!