Forgive me if I must burst a bubble or two. You should read and learn a lot more about microscopes and microscopy before proceeding to spend any money.
Your notion of employing 400x with a compound microscope would be far, far too great a magnification for you. One, you’d have to have the objective of the microscope impractically close to the surface you wish to inspect. Two, adequate lighting would be problematic if not impossible. Plus you’d be hard-put to understand what you’d see at such unrealistic magnifications. Same for a stereomicroscope at 180x.
In short, your proposed magnifications are totally unrealistic.
A stereomicroscope with good lighting would be my first choice. Magnifications of 20x up to 40x probably would be most useful. But as with the compound microscope, critically lighting the object will take skill, practice, and an understanding of what you are doing.
To address your closing question:
The objective provides primary magnification of the object being viewed; the eyepiece (ocular) magnifies the image obtained by the objective. The information gathered by the objective is the principle limiting factor. Adding magnification at the eyepiece cannot add any additional information about your object, it can only magnify what comes through the objective. Adding magnification at the objective can, by contrast, give you more data (information) about your object.
What you want is good resolution so you can see a useful amount of detail. Bear in mind, too, that part of the process is learning to see, to observe, just as it is in astronomy. You’ll also need to learn about optical glass surfaces in order to understand and interpret what you are seeing.
Have you considered starting off by obtaining a very good loupe (such as Belomo) offering about 10x magnification ($45 USD)? Even here, lighting is a critical factor in obtaining a good, detailed view.
Sorry to disappoint you!
CS and good luck!
Edited by Brianm14, 13 September 2022 - 12:14 PM.