I wouldn't be too worried about getting the mesh size exactly right to start with, because it's only for small batches it's easy enough to sift and rerun the stuff that didn't crush fully. So probably 3mm mesh should be fine, but it's going to depend a lot on the machine, the rock being processed and what the user needs. Ideally you could make it with a very wide outlet mesh(10mm), but allow the user to cover that with a fine mesh of their choice (yeah l know it's all extra work).
The inlet angle would likely be fine at 120 degrees, but lve never built a crusher from scratch, lve only ever modified them. Do a search on crushers in this forum and you should find a huge amount of information about what does and doesn't work and why. At 90 degrees rock just gets stuck a lot, even stuff you would think would be fine.
Even with all the stuff lve been saying, you look to have a good little crusher and though l can see possible improvements, it should certainly do a decent job and l do think the price is reasonable, even with the postage.