Maybe not the kind of answer as it’s just technical data but to compare SNR (Signal to Noise Ratio) (noise/ISO) between camera sensors, the DXOmark database is useful I think especially in the context of your demand.
https://www.dxomark.com/Cameras/
To be noted that it compares the SNR is “per pixel” and that a higher pixel count sensor with the same ISO/noise performance will produce an as intense but finer noise wish is less detrimental to the overhaull image quality.
For comparison :
- Lumix GH1 12mpx 772iso to reach targeted SNR threshold in the test
- Canon 50D 15mpx 696iso " "
- Canon 80D 24mpx 1135iso " "
- Sony a7 24mpx 2248iso " "
- Sony a7 III 42mpx 3523iso " "
The database will also give you release date and msrp wich can be used to “guesstimate 2nd hand price” to narrow your search.
Personally I switched from Lumix-GH1 (m4/3 12mpx rated 772iso to the SNR threshold in dxomark) to an Sony A-7 (full-frame 24mpx rated 2248iso to the same SNR threshold) using the same vintae lenses via adapters so the upgrade was quite dramatic !
On some play raws I found out that the a7-III was particulary good at the “picture hidden in the dark” game but it is not in the same league as a 80d or an a7 in term of price …
In my case this is by using this tools (and dpreview to check other features) that I ended up buying my A7 2nd hand for 250€ 2years ago.