creating a BIG membership site (30K members)

I’m building a membership site expecting to have around 30,000 members. So I need to build it in the most solid was possible.

I’m so VERY impressed with the Kleo theme. Thank you! I’ve had no problems at all so far.

I just need to know if you see holes or vulnerabilities in this plan:

WP Engine hosting (I installed that special code in the child functions.php specific for WP Engine), Memberpress, Buddypress, bbpress. All paid members will have access to the forums (12 to start and there will be subforums from there). All paid members will also have a profile and be automatically signed up into a group with 25 other members. So we’re talking about 1000+ groups eventually. The groups are there to support each other in a more intimate setting than the forum and admin posts to the site.

Spam issues: as far as I know, Memberpress covers this with 3 levels of spam protection. Is this correct?

What other types of systemwide issues could come up that I should prepare for from the beginning? Obviously with that many members I need the site to be secure and not crash, etc.

I greatly would appreciate a detailed answer. Again, I need to do this right.

With much gratitude,

Jen