You can use whey from any milk to make ricotta , but some cheese styles will leave very little curd left in the milk.
Good meters will hold calibrating longer than cheap ones in general , my Hanna can be off .02 after a month or so .....hardly worth worring about , my last meter would clog as I used it during the make a start to give wildly inaccurate readings .
If cost is a concern you can buy little sealed foil packets of calibration fluid on line , cut the top corner off , pour a little in a bottle cap or some other small container ( this fluid can not be put back into packet ) so the smaller the container the better . This will be good for the whole make session. Fold top over on foil pack and seal with clip , I stored mine in fridge back when I used them.
Ph cleaner is also important ..... not sure if it comes in foil packets or not.