First let me congratulate you on your engagement. I'm recently engaged myself and I'm very excited and I imagine that you are too.
1. I feel it depends so much on the maturity of the individuals involved as well as life experience. By that I mean has he and she gotten all of the dating and so on out of their system that they wanted to and can take their vows seriously? Have they accomplished the goals that they wanted to accomplish PRIOR to getting married? Marriage definately can change priorities in some cases and accomplishing "individual" goals can be a little harder when married.
2. As for asking the father's permission...I guess that's nice if you want to be old fashioned/traditional, but if you really want to get married you are going to do it regardless of what daddy have to say. I know that I would, because at age 27 I am clearly grown and have been for a long while
. It really is a nice gesture and a sign of respect towards you and your family, but in my personal opinion I don't feel it's extremely neccessary. Formal announcements and all of that is up to you. My only requirement is that it's at least white gold and not yellow gold.
3. Diamond engagement rings...they are nice, but that's up to you and his bank account
. I still haven't received my official engagement ring yet (my proposal was crazy), but I'm not concerned with having a diamond either. My fiance is fortunate enough not to be concerned about the cost, but I personally want something simple and with an antique feel to it. The diamond (for me) isn't neccessary. I can have the biggest rock in the world, get jacked, and there goes my diamond...but I will still have my love for him which isn't tied up into a ring...big or small.
Good luck to you and enjoy planning your wedding.