It only has what you want as you have to install all of the programs that you want which gives you the most flexibility.
It is a source based distribution which means when you install something it compiles it.
As for TWM you can install it using portage which means it should work out of the box.
Here is the install Guide
This option gives you the most combustibility but takes a while to setup. Though you get a nice speed increase after everything is installed and portage takes care of all most all of the hassle associated with compiling and dependency issues. You could even set it up to compile everything without making it specific to you computer and you could then "drop" the installation into just about any other computer with the same arch. Rebuild/configure the kernel and it would be ready.
The hardiest part it the setup/installation after that the benefits are incredible.