There is some discussion of the condenser lens and alternatives in other threads, including Suitable condenser lens "beam angle".
The f=50mm achromatic lens is usually a bit easier to find, although it can be relatively expensive. It is more of a standard part for scientific experiments, so you would be looking for scientific optics suppliers.
If you build the ‘low-cost’ optics module it does not need the achromatic tube lens, which means there are fewer parts to find. The field if view and resolutikn are similar to a 20× lens on a conventional microscope. This can be a really good place to start. To move over to the high-resolution optics later you just need to print the optics module and assemble it with the tube lens and objective. Everything else can be re-used, including the camera.