I have only limited experience with OpenACS on OSX thus far, and from what I've read and played with, you should be able to accomplish all of your goals and more with OpenACS 4.6. The unix install guide will help you get OpenACS, PostgreSQL and AOLServer installed. I'd recommend reading through the rest of the documentation too, especially the developer's guide.
Take note that OpenACS 4.6 does not yet work with PostgreSQL 7.3.x, just PostgreSQL 7.2. I just downloaded the 7.2.4 source and compiled and installed that, but there may be binary packages available somewhere. For AOLServer I just used the prepacked tarball with all of the goodies that is mentioned in the install guide. I think there were a few snags here and there (having to comment out #includes for crypt.h, which is not needed in OSX, etc.), but it was quite trivial to get it all up and running on my PowerBook (pismo, G3), and it runs good enough for testing/development on it.
