Quite a few of my customers are in "rural" areas with wells. Here are a few issues I deal with:
1. Hard water deposits on the windows. Let the customer know, or you'll be washing windows by hand.
2. If the customer will not be there during the cleaning, ask them to allow you access to the pump reset!
3. If you haven't actually tested the gpm's, I would do it. Aside from the damage to your pump, it will take you much longer to do the work. If there is less gpm's than you need, get a tank. Even a 55 gal drum would be enough. If your pump is 4 gpm, well is 3 gpm, then you'll have 50 or so minutes of non-stop (assuming you fill the tank) cleaning. Make sure the drum is above your pump.
4. If you use a tank, discard any leftover water - you won't want to use it anywhere else.
5. Ask the customer if their spigots are connected to there water softener. If it is, there will be fewer spots, but they may not want their systems overworked.