Moonshell is not required. Yes, you can download the latest version and install it on the Simply. Just download it, extract it, run the installer, and select the R4 DLDI driver when it asks.
If you want Moonshell to start when you activate the middle button on the Simply's menu, you have to rename the .nds file to "_DS_MSHL.NDS" and place it in the root of the microSD card. If you want the middle button to start a different program, rename the .nds file for any other homebrew app to "_DS_MSHL.NDS" and that app will start when you activate the middle button.