you are right, the temp of the reservoir is crucial but not indicative of a bubbler system, nor an ebb and flow, or drip system. In other words, your reservoir should have cool temps no matter what system you use. Also, ice / 2 liter bottles are an excellent way of cooling a reservoir IF you have temp problems. Low 80's in summer is not that big of a deal. Your res should still stay in the 75F range and that shouldn't be a problem. Not only will temps cause your pH to fluctuate but your pH meter (depending on what type and quality) will also fluctuate based on the temperature.

There are numerous commercial pH buffers on the market. Visit a hydro store or shop online. They come in crystal and liquid forms.