Yes, you can easily (there are templates) put ads between posts (and even restrict who sees them with conditionals)
[ATTACH]125984[/ATTACH]
Not exactly sure what you mean by this
Are you referring to single sign on ability?
As for the second question about show/hide things... yes, again a lot you can show/hide based upon conditionals in templates. Some things may require an add-on. The question is very broad so it's hard to answer directly.
As for CMS, there is only one add-on that bridges WordPress that is even worth considering from what I understand - but I've never used it.
Unlike IPS, you can (as long as your license is not suspended) download the latest version available when your support expired - and further, unlike with IPS, you can still access the Resource Manager and continue to download add-ons even if your license is not "current".
As for XF2 - from my understanding, it will be a free upgrade, not requiring payment for use of it - no different than going from 1.4 -> 1.5.
