I would spend the extra money and get a good one. Cheep china stuff dosnt work right or last long if it dose. But if you get that don't use the light measurement. Don't see how it will be true. And you don't realy damage that many roots putting probes in the soil. So no don't leave it in the pot.
What soil are you using? I've never used a soil ph tester before and don't see a reason to. As long as you use good soil made for cannabis and ph your nutrient water than its fine.