Something you should all be aware of on pH testers is they don't last, especially if the tip dries out. Also, you need to get calibration solution, which also can dry out. The more expensive ones use replaceable tips, but the less expensive ones are now cheaper than the tips.
For beer the resolution and accuracy does not need to be as good as for wine. A pH of 5.2, 5.4, or 5.6 are all fine for beer, where as in wine, you need to be sub 0.1 resolution and accuracy to do the best job. So do an ebay search for Hanna (one good brand) and see what comes up. Also consider test strips. You can probably get away with test strips with beer, and once you know you are in the range, you don't need to test every time.
Also, consider getting your water tested, then use th's spreadsheet "EZ_water_calculator" over in the brew science forum section. It works amazingly well at predicting the right water additions to get your mash pH to work. It's one of the final tools needed to make great beer and have reliable mash efficiency. I haven't even bothered testing my pH since using this tool.
You do need a reliable water source that doesn't change chemistry much. (I only mention this as one municipal source blended water from different aquifers at different times of the year to meet EPA regulations)