I am also shopping for few new oldie cameras i found this list about write speed
Write speed for some Canon cameras
D30 – 1,2 MB/s
D60 -1,8 MB/s
300D – 1,4 MB/s
10D – 1,4 MB/s
1Ds – 2 MB/s
1D – 3,3 MB/s
20D – 6 MB/s
350D – 6 MB/s
30D – 6 MB/s
40D – 11 MB/s
60D – 21 MB/s
600D – 21 MB/s
550D – 21 MB/s
500D – 21 MB/s
1100D – 21 MB/s
EOS M – 44 MB/s (Overclocked at SDR104 @ 160MHz: 69 MB/s)
EOS M2 – 44 MB/s (Overclocked at SDR104 @ 160MHz: 69 MB/s)
1200D – 44 MB/s
100D – 44 MB/s (Overclocked at SDR104 @ 160MHz: 71 MB/s)
650D- 44 MB/s (Overclocked at SDR104 @ 160MHz: 71 MB/s)
700D -44 MB/s (Overclocked at SDR104 @ 160MHz: 71 MB/s)
70D – 44 MB/s (Overclocked at SDR104 @ 160MHz: 71 MB/s)
6D – 44 MB/s (Overclocked at SDR104 @ 160MHz: 71 MB/s)
1D IV – 66 MB/s
50D – 70 MB/s
M50 – 80 MB/s
5D II – 85 MB/s
750D – 90 MB/s
760D – 90 MB/s
800D – 90 MB/s
77D – 90 MB/s
80D – 90 MB/s
7D – 90 MB/s
1Dx – 120 MB/s
7D II – 120 + 90 MB/s (CF+SD)
5D III – 120 + 21 MB/s (CF+SD) SD card write speed will limit CF slot write speed if you use CF + SD at the same time. (Overclocked at SDR104 @ 160MHz: 51 MB/s)
5Ds – 120 + 90 MB/s (CF+SD)
5D IV – 130 + 90 MB/s (CF+SD)
EOS R – 200 MB/s
here is the link
https://kameratrollet.se/canon-write-speeds/