Ph pens need calibrating to be accurate, and that requires calibration solution. Also, you can't let the probe dry, or it will kill the effectiveness and accuracy. Water can be used, but it spoils and becomes an algae farm. Avoid soil probes, as every one I've tried sucks bad.

I get my pots from the local nursery, where the used pots range from 25 cents US for a 1 gallon (4 litre) pot, up to one dollar US for a 15 gallon (60 litre-but too big for me) pot. I flower in 5 gallon (20 litre) pots, though. Pot's are only new once, so used pots are fine. If concerned, they can be washed-out before use with bleachwater.

I'd have to know the soil ingredients before commenting on it's effectiveness. I work with peat-based potting soils, and nothing else. Works great.