"how important is it for me to be there for the survey on the hard?" (The broker will be there.)
I think it's a really good idea to be there. The broker is interested in closing the sale, so he is at best a neutral party to the process.
When you get your nine page report, there are going to be things in it that you may not understand. There are almost certainly going to be recommendations regarding upgrades to current ABYC standards. Some of those are important and some of them are not as important, but your surveyor is not likely to differentiate between the two.
As well, he may note things that you do need to take care of, and sometimes it is difficult to grasp what he means unless he actually shows you. Particularly as it looks like you are going to be using a different surveyor for the sea trial (why ???). The initial surveyor may have questions about something that can only be answered when the boat is in the water, but the only person who is going to know what to look for is you.
Unless you are 100% confident that the boat is in great shape, and are willing to roll the dice, you should attend.