Will GNOME Shell need compositing (like Unity does)?

Both gnome-shell and Unity will require compositing in order to be used. You can install proprietary drivers and still have it on a USB/external drive - I do it all the time and move from computer to computer. Besides, you can always install the Gallium3d drivers for Nouveau and you'll have fully-supported 3d on all the important cards.

Both Unity and Gnome-shell are anything but out of the question.

Yes, it will need compositing, however there is a fallback. From the GNOME3 FAQ:

Computers purchased in the last 4 or 5 years should be more than capable of running GNOME 3. The GNOME 3 desktop does require hardware accelerated graphics in order to provide a cutting-edge experience however, and the complete GNOME 3 experience will only be available on computers capable of this. Do not worry though: GNOME 3 will come complete with a fallback interface which will provide an excellent experience in the absence of hardware acceleration, and which incorporates many of the improvements that can be found in GNOME 3. Furthermore, the GNOME project and its partners are working hard to ensure that the complete GNOME 3 experience is available to as many people as possible, and aim to ensure that users who are initially unable to have this experience will be able to in the future.