Thanks for the reply!
Im doing an internship at a calibration company(Teknological Institute), so it doesnt have to be cheap - it "just" has to be good!
Right now we have a Fluke Hart 2100.
I was looking at 24 bit ADCs or maybe even 32 bit ADCs. Is it possible to get the needed resolution if I connect the ADC directly to the PT100 or should I put some kind of an amplifier in between the two? Do you know if it is possible to buy a package with current source and ADC made for the PT100 sensor? My plan was to take the ADC output into a µController and further on into a PC.