Hi Bart,
For what you are planning to do even 8Bit would be fine. As the ADS1115 is so cheap and versatile I use it because it gives me everything I need (and will ever need). 15€ for a pack of 5 from a bigger German supplier (china is becomming less interesting in the last years with higher shipping fees - so support your local heroes)
As you do your research right it is worth to take a look into the datasheet (always a ton first hand information)

Although for your application the ADS1115 might be a total overkill it might be beneficial to get familiar with one ADC which you can use with every micro (and the PI) on the market.
But As I said the internal ADC is fine too (and one component less) and a zener diode (one component more to specify and source) as @dynamicdave suggested is fine too.
Again a quick read in the datasheet about the analog input pins:

So you run your ADS1115 on 5V you can scroll (or follow the link) up and read under 7.1

Your maximum Anlaog Votage is 5V + 0.3V = 5.3V! I can't see how you will be able to exceed this with your circuit. (and the minimum is -0.3V)
I don't see any need of more protection (more components). If you check your circuit before powering on there is no potential problem other than human error. (BTW: ! have 20+ ESPs and only fried one by inserting it 180° reverse into the socket of my motherboard - shit happens)
Last but not least as there is a lot of controversy on the net about 5V inputs pins on an ESP: Is ESP8266 I/O really 5V tolerant? – Digital Me
But this does not apply to the one ADC of the ESP8266. It works from 0-1V of the chip. Most boards come with a build in voltage divider to allow inputs from 0-3.3V. You have to take this into account when designing your voltage divider for ESP8266 boards. One Resistor to widen the range might be enough.
Simply speaking: I run 5V i2c (digital) buses on 2 devices since 3 Years with ESP8266 now with no problem. good signal stability (perhaps better than on 3.3V)
As i2c is a bus all your devices on one 5V bus have to be 5V tolerant. But you can have multiple i2c buses as i2c is a so basic protocol that the arduino drives can emulate i2c on all I/O Pins via bit banging in software (think they do this anyway even on the official D1/D2 pins). So @dynamicdave circit is absolutely correct running on 3.3V as he has a BME280 on the same bus.
Last but not least why not throwing a couple of DS18B20s in the mix to measure some temperatures of your heating system

They run on the so called DALLAS 1Pin Bus with individual addresses for each sensor. BUT: This is again a bus topology! Running many in a star configuration is at least unreliable. If you have cables longer than lets say 30cm you better connect each on its own I/O Pin (but there are some left over as your 4 ADCs only use 2 I/O pins 
The packaging is easy to strap on pipes or simply pushed under the isolation. And you even could drown them into water (I have 3 in my pond for years one at -1.5m)