I love my Clack. I've got a 6x9 version, but not such a great deal because it's mostly blurry on each side anyway! No worries on the film, these were made when "high-speed" film was 100asa. Since you are shooting slide film, just make sure the sun is behind you.
One thing to mention. There is a red window with a cover so you can see the frame numbers when you wind. Older film was red-light safe. Go into the shade to peek and wind to the next frame.
Just thought of one kinda neat thing about the Clack that you might use to creative effect. The shutter cocks and fires in one motion, meaning you can do some interesting motion stuff quickly by click-click-click'ing as a girl on a bike passes by, or suchlike.