Retroboy
for graphics i would suggest a used gtx 750 (ti)
you should get one for around 30-40€
for cpu the minimum for gamecube would be a haswell pentium or i3 and for ps2/wii a haswell i5 or i7
they are also sold very cheap at the moment (i saw haswell i5 cpus for about 40-50€ in market place of several hardware-forums in the internet)
that is from my experience so far (intel/nvidia is a bit better supported under Batocera than AMD/ATI)
as a l ittle orientation help / guidance i recommend to have a look at passmark cpu/gpu mark for comparison of the performance of CPUs/GPUs
https://www.cpubenchmark.net/cpu_list.php
https://www.videocardbenchmark.net/gpu_list.php
i would recommend a CPU with at least 1.500 in single rating performance and a GPU with at least 2.000 G3D mark for gamecube.
For PS2/Wii you should have a CPU with a single thread performance of at least 2.000 and GPU with at least 3.500 G3D Mark (but that is just a guesstimate)
For an older batocera version (a beta of 5.21) i had made some tests with all the PC-Hardware i had at home (i sold many of them in the meantime):
https://docs.google.com/spreadsheets/d/1K-HDv3SXqWj-aoniuAJnFzsRNsg8HyHv55C0RJgFgrY/edit#gid=0
0 means the game did either not run, or run slowly (not fullspeed)
1x means that the games run fullspeed (30 or 60fps) in native resolution.
2x means 2x native resolution
3x = 3x native resolution
and so on