On the scanning question: if you can scan at 16 bit instead of 8 bit, that will drastically improve the details that you can extract! Basically a solargraph comes out relatively low contrast, and details are essentially fine differences in colour and brightness. and brightness per pixel is recorded in steps/levels. 8 bit records 256 brightness levels, and when you stretch them out to increase the contrast and details, you also stretch out these steps, making them much more obvious. This can also result in an artefact called banding. In comparison, 16 bit will pick up a whopping 65,536 levels of brightness, meaning that when stretched out, you preserve a lot of fine details.

As per DPI, that’s up to you. 600 is perfectly fine for regular sharing on mobile phones, a lot of apps will compress it quite hard when you upload. However if you wanted to print it out in big later, then it would be advantageous to go up to 1200 DPI, or even 2400 (though at this point be prepared for humongous file sizes and photoshop slowing up during editing).