Hi Malte, the installer does indeed work in OSX, we will be releasing a version over the next 2 to 3 weeks. I can probably provide you with a beta build earlier than that.
Creating a simple installer (copy files, create shortcuts, etc.) is going to take you less than an hour. But in your case, it is going to take you a few days, not so much because the installer itself but because for your all-in-one installer you will be shipping a number of components that need to be built, made relocatable, etc.
Re: some of the tools been already on the system, my advice is that, for a first release, you bundle everything you need in the installer. This will simplify the code of the installer and will give you a controlled environment that will reduce the number of installation issues due to different versions, etc.
On later releases, you could add custom screens and actions to the installer (they can be defined easily in the project.xml file) that autodetect or ask for the location of the different components that may already be installed in the system.
A different approach, used by many of the last generation CMS is to use the installer to install a basic OpenACS system, start it, and then use a web-based procedure running on OpenACS itself to do all the setup (where is Postgres, what is the admin password, etc...)
