I’ve been working towards a similar goal to you, and have mostly the same solutions. Developing a stream monitoring system in South Africa though. About the electronics: I’m not sure if you’ve considered this, but the normal Arduino boards draw quite a lot of quiescent current, due to the onboard regulator, USB etc. Are you looking at using an Arduino Mega Pro, from Sparkfun perhaps? Or, have a look at the Seeeduino Stalker; it comes with a built-in SD slot, RTC and Solar Charging circuit, which makes it pretty great for logging. If you use one of these, the neat thing is that you would be able to run your entire unit off 3.7V, so you wouldn’t need the three cells in series. It’s far more efficient, especially since the regulator on the UNO/Mega is not particularly energy efficient.
When it comes to the turbidity sensor: they should definitely work. I know some guys in Europe have used them. I just am not familiar with the type of output, so I wouldn’t be able to point you to a circuit offhand. Do you have a datasheet for it? I might be able to help you then!
Also, bear in mind that that particular turbidity sensor is an attenuation sensor, and is generally not advisable for stream monitoring. It can easily be affected by water colour, for instance. Of course, if the cheap option is what you need, it is very useful :).
With regards to maintenance, it depends on how well you design your system, but the probes will need regular calibration. I know Atlas Scientific claims that their probes (which are designed to run easily on Arduino, by the way!), only need calibrating once a year. My concern would be your nitrate sensor, which only has an expected life span of 6-9 months.
I’d love to hear more of what you’ve done so far. I’ll take some more time to properly draw up what I’m doing here soon!