You will definitely want to see also if you can charge the keyboard and mouse. Battery life is typically good for 2-5 years of use. Sitting on the shelf for over 3 years, you may need to have an authorized service center replace the battery. You can charge the keyboard and mouse with the same charger used for an iPhone, as long as all it has is the standard USB 3.0 or earlier port on the charger and the cable the came with the keyboard and mouse. Most of those wireless keyboards and mouse from Apple supported Apple's lightning cable.
The important thing is the white portion on the tip as shown on the cable end on the right side of this image. If after charging the mouse and keyboard overnight, they still do not show any sign of life, you'll want to replace the battery or the keyboard and mouse with another one.
I do agree, getting a wired keyboard and mouse is the first step, to walk through setting up the Mac.