Based on what you just said, I strongly recommend sticking with Wordpress. If you have a hard time finding specific plugins to fulfill specific needs you are having, please let us know. There's a plugin (or twelve) for pretty much everything under the sun, but quality varies pretty dramatically. Always a good idea to double-check with the Wordpress community (which happens to be well represented on this list).
As far as AJAX goes: lots of folks use it (on pretty much all the CMS platforms), it isn't always necessary, and without some more specific context, it's hard to make recommendations.
It sounds like you could write your own plugins in a pinch if need be? PHP and JavaScript help can be had here as well. The more specific your question, the easier it is to answer.
-Jason