@adrian what type of things are you going to be looking at, or what magnification do you need? The Basic optics module is remarkably good and does not need an objective lens or the f=50mm tube lens. It will have similar performance to a conventional microscope with 20x objective and 10x eyepieces. Threads here: Comparison of a pi-camera and a 20x optical lens - Contributions - OpenFlexure Forum for a couple of pictures and Openflexure Microscope High Resolution vs Basic - Request Help - OpenFlexure Forum.
You can always build the high resolution optics later and move the Pi Camera over.