Dynamic range is usually not something quoted for A/Ds, and it's ambiguous what exactly it might mean. A/Ds have a fixed word width they convert the analog signal to. With N bits, there can only be 2^N diffent output values. You could say that "dynamic range" is therefore (2^N):1, but that is somewhat artificial.
You are correct, however, when you say you are confused. Reading a good general introduction to what A/Ds are and how they are usually specified would be a good start. Another way to learn this is to look at some datasheets from a sampling of manufacturers. After a few you will see what is specified in what way.
You say the design goals are reliability and simplicity. Reliability without quantification is not a design goal or specification. It is one parameter that can possibly be traded off with cost, size, power consumption, and other parameters. Without knowing the relative values of these other things, simply stating you want reliability is not meaningful.
There are several different overall methods of converting a analog signal into a digital value. Some examples include successive approximation, delta-sigma, tracking, and flash. Each method has its own set of advantages and disadvantages. For example, delta-sigma can have very high resolution (20 bits or more) but is slow. Flash is very fast but usually more expensive, power hungry, and often suffers from larger non-linearity errors. You have to start with the characteristics of the signal you want to measure and how you want to use those measurements before you can pick a A/D.
This is not for you
On a separate topic, you have no business designing something where "the results can endanger someone's life" (your words) if you have to ask very basic questions here. It may be legal for anyone to work on such a product, but eventually there will be testing done with strict legal limits that need to be met. These include safety features such as minimum isolation distance, maximum leakage current, and various others. And that's only the beginning if this product can endanger someone not just thru shock, but by providing incorrect readings and the like.
You don't say where you are located (that's really for us, not you, so it's kind of rude to leave it off), but here in the US the FDA regulates such things. Not only does your product have to conform to numerous regulations, but so does your design process. If you don't know what a "510K" is, then you really really don't belong in this situation. You can assist a senior engineer and maybe next time you can take on a more responsible role yourself, but do yourself and everyone else a favor and bail out of this trap now.