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Hi Torsten,
as other have said the Point Grey cameras are a good place to start. They used to have a great guide where you could see the sensitivity treshold for all their cameras but now they are FLIR and I can´t find it. I have a copy from 2017, it may be useful-PM me if you want a copy (you can also navigate the FLIR site, which is more time consuming but you have all the new models. You can also find those cameras at other retailers including Edmund. )
Clearly they are not scientific (quantitative), but they are sensitive and are very helpful when trying to find a sample and so on. I have tried a few, and the numbers don´t always tell the whole story, sometimes there is a lot of salt and pepper noise. We have had a good experience with their Pregius sensors (IMX249 as mentioned previously). Several groups have used different pregius sensors to do super-resolution (palm-type) microscopy, and their cost goes from ˜500 to 1500 USD. Depending on what you need in terms of speed, magnification/pixel size, FOV, cost and sensitivity, different versions may be best for your application, and the read noise tends to be around 2-3 electrons, with a quantum efficiency ˜70%.  One nice thing is that there is a Micromanager driver , so it is easy to integrate into whatever microscope setup you have  https://micro-manager.org/wiki/Point_Grey_Research. You can then use micromanager connect the camera with Matlab. 
If you need long exposures there are even cooled versions that are used in amateur astronomy (highpointscientific) but I am not sure what the driver situation is for microscopes, and there is even an sCMOS version of the camera that I heard of (Spot imaging), but I am not sure about the cost or have any experience with it; would love to hear if anyone has worked with it. If you want be quantitative and not spend extra money, I seem to remember that one of the papers that did superresolution had some software to linearize the output, if that could be integrated into micromanager it would be a great contribution to low cost science. 
Best,
Manu

Manu Forero Shelton, Dr.Sc. 
Associate Professor
Biophysics Lab, Department of Physics
Universidad de los Andes, Carrera 1E # 18A - 10
Bogotá, Colombia. Tel: (+57 1) 3394949 x2621