Investigating a factor affecting buffer capacity
As far as I can tell, something like temperature won't affect buffer capacity.


In this particular case, I wouldn't be too sure about it. Isn't it conceivable that your buffer will lose ammonia upon heating?

Did you already decide about the pH indicators? For the lower end, thymol blue might be a good choice. The supplementary data
to J. Mater. Chem. B, 2015,3, 2801-2807 contains a UV/VIS spectrum of this indicator.