You should try the Ashida. It has a convenient BOM (Bill Of Materials) which goes over everything you need. there are tones of worklogs about it and you only need to order from a few places to accumulate all the parts you need. It also uses custom PCBs for stuff like the controller and the interface, making it the perfect choice for a beginner making their first portable. Here's the URL for the thread:
Ashida Wii Portable | BitBuilt - Giving Life to Old Consoles
the G-boy isn't as good to get started anymore as the kit isn't available anymore and Gman hasn't open sourced more than the case files, meaning it isn't as good a starter as the Ashida. it does have an updated version that uses the 4 layer tech boards, but it is more annoying to get all the parts for it. URL:
https://bitbuilt.net/forums/index.php?threads/g-boy-pcb-files.5054/
Another option is the G-Wii. it is smaller than the Ashida, but doesn't have those magic custom circuit boards, so therefore also requires more wires.
(1) Model - G-Wii CAD files | BitBuilt - Giving Life to Old Consoles
The Louii is another portable, but it isn't recommended as it is quite a tight build and requires a more advanced version of the basic OMGWTF trim. It also requires you to order custom circuit boards from a manufacturing company like JLC PCB, PCBway or OSHpark. cons aside, this is a very nice, sleek portable, smaller than the G-Wii and the Ashida but with the same 5 inch, 16:9 IPS display and with a BOM. it is smaller than a switch with the same buttons and joysticks. URL:
(1) Model - Louii Twoii CAD Files | BitBuilt - Giving Life to Old Consoles
Hope this was helpful!