With respect to the 401k, many companies automatically enroll you in the plan up to the matching amount. You can opt out if you want, but you will lose the match, which is one of your benefits.
Agree on the dress code - wear a suit the first day and figure things out fromt here.
Tell them about the June wedding. Don't worry about the hours just now. Tell them about November when the time is appropriate.
Congrats on the job! I would send in your letter now - shows that you are excited about being a part of the company.