River testing kit (TMP, pH, DO, EC)

My school spends around $4000 for devices that can provide all these measurements, but I think an Arduino based solution is probable

Here are the kits/sensors I have found for this, mostly from Atlas:

TMP: Page has moved or https://www.atlas-scientific.com/product_pages/sensors/env-tmp.html
pH: pH Kit | Atlas Scientific
DO: https://www.atlas-scientific.com/product_pages/kits/do-kit.html
EC: https://www.atlas-scientific.com/product_pages/kits/ec-kit.html

My question is:

Would one Arduino suffice for a project like this?

and subsequently:

Which Arduino?

and also:

What kind of circuit (generally) would allow for something like this? I'm thinking of output to an LCD that can be switched between modes (TMP, pH, DO, EC)

I am a beginner but I have been waiting for an idea for a big project and this seems perfect.

just another choice

The boxtec DS18B20 is about three times the price you would pay on eBay.

A Uno would suffice but a Mega would be a better bet as you might want to take the project further with internet connection, or data recording, so you might as well pay the extra $10 and get one in the first place.

A 4x20 LCD is an obvious choice, no switching needed.

The two big problems are the ones you haven't talked about - power supply and communication, where is the river? And they might explain a little of the $4000 currently invested.

One Arduino could suffice, but the communications issue may mean it is simpler, and cheaper, to use two.

I have been waiting for an idea for a big project and this seems perfect.

It probably is,and you will find plenty of support here