pH Strips 4.2> 6.2, 100 pcs
A reasonably good pH meter easily costs more than 150 Euros and on top you need calibration fluids. Most pH meters have a limited service life - often only 1 year, regardless of whether you do your best to clean the electrodes.
The alternative is pH Strips with a narrow range that covers the area you want to measure. While the method is somewhat more imprecise and can only be read within +/- 0.2, this solution is also cheap in daily life. If you check your water twice a week, you have an entire year's consumption!