At this point, all you need is the back motherboard from a beaten up Game Boy and you can get a fully function unit for about $100. The front daughterboard, screen, power board, sound board, membranes, buttons, case, stickers, and lens can be bought second hand, and in good quality. Then you can add on USB C rechargeable batteries, sound amplifies, AV out (for GBA and above, for now)... Every few months a radical new mod comes out.
My only concern is that modders might be tempted to pull out old mods to install slightly upgraded versions. We're talking about 30+ year old devices, and yet the mods are going out of date.