You need to ask yourself couple of questions before you can select best meter for you.
1. How accurate measurement do you need?
2. How large is your sample size?
3. What conditions are you making measurement in?
Here is why it matters:
1. If you are just looking for a large drift in pH ,you probably do not need a measurement accurate to 0.01pH. If you have a spec sheet which gives acceptable range of +/- 0.1pH then you probably want a meter that is 0.01ph accurate (this will cost $150 for reliable one).
2. If your sample size is just couple of drops, then you are kind of stuck with ph paper. If your sample size is like 50ml then you can actually use real meters.
3. If you are doing the measurement outside in bright sunlight or in dark and dim basement will determine how much PITA it will be to use color driven ph tests like ph paper.
After owning a $20 ph meter that supposed to read 0.1pH accuracy and then owning a $30 ph meter that supposed to read 0.01pH accuracy i ended up buying a $150 pH meter that rads 0.01pH accuracy and could not be happier. You might be able to get away with using cheap meter (potentiometer calibration) for a ballpark measurement if you calibrate it at Ph close to what you are measuring and you always measure small range of pH values, say 6-9pH. Also, if you are going to go the "cheap potentiometer based meter route" consider pH meter with 2 potentiometers, one for slope and one for offset. You really need to adjust both to get a decent reading over a deently wide range of pH values.
Also, consider the temperature of the coolant. Unless you have a coolant sample sitting comfortable on a table at room temperature for all your tests, you probably want a temperature compensated pH meter; otherwise you can fool yourself into getting wrong / changing values due to variance in temperature of the test sample.