Advise and suggestions rebuilding website.

Hi,
I am new to WordPress and I have a few websites built using a different CMS and want to migrate them all to WordPress.

Between google and youtube I learnt about WordPress widgets, menus, plugins and themes and recently rebuilt this site https://aonbuild.com/ from the old website https://www.onipede.com/aeon/

The next website I want to migrate is https://www.gti-taekwondo.com/main/news.php which is a lot more complex. I need advice and suggestions on how best to go about it specifically –
1. What plugins would you need recommend for an internal forums system, calendar, gallery and documents downloads
2. I need to add user with different access level, i.e some can read only; others can read and post. Ability to view or download documents, edit pages etc
3. A twitter menu
4. Page visible to different user classes (I think this is easily set on each page)
5. Means to manage 59+ site links easily
6. Event calendar that certain user classes can create event – Forth coming event menu.
7. Theme suggestion (I know this is subjective but helps if I have a few suggestions, will are willing to use commercial theme) Mobile option??

Also do you have any suggestions on how to swap from old site to new site with minimum down time. I was thinking of setting up a sub-domain called wp.gti-taekwondo.com or similar, then once I was happy with the site, change the domain to the primary domain following the instructions laid out in https://support.godaddy.com/help/article/6495/changing-your-wordpress-domain-name

Any other suggestion?? I am not in a hurry to make the move so willing to soak in advice first. Using a Linux server on Godaddy