Dynamic Range of an ADC
This should be answered formally by someone who you are paying and who signs his opinion and can be held accountable for it - or ata least who specifically states their qualifications, competence and degree of culpability in the event of subsequent "issues". 

There is more to this than can be answered without a deeper level of understanding than your abstraction will allow and than can be even partially answered well, without many to and fro q&amp;As, in this sort of forum. We'll try regardless :-). 


  " ... the more reliable the output (ie closer to the analog input). ...


Your use of the term "reliable" is non standard and demonstrates a lack of understanding of the process you are attempting to monitor. This is not meant to be rude - just factual. And important. 

If you are making this instrument then somebody with a good degree of technical  understanding needs to go over the proposal and design in detail. If you are buying it then the name on the nameplate is more important than the specs of the ADC. eg you can be about certain that if it say "Agilent" (and is genuine) then it will do the job well and be 'reliable enough' [tm]. If it says "eg "Golden Sparrow" you may wish to look elsewhere. 

ADC resolution affects the potential accuracy of the result. As the required resolution increases the other factors which affect achieved accuracy and resolution become increasingly important. 


Example: I have a well built but low cost digital scale with a full scale reading of 500 grams and a resolution of 0.01 gram (10 mg). I can obtain about 300 mg of reading variation by holding my wristwatch about 30mm above it in one orientation and about half that when the watch is turned at 90 degrees. An electric fan heater with coiled heating element of about 150mm dia, when operated  within a 100 mm of the scales turns them into a random number generator).   


ADC technology influences susceptibility to external interference. Successive approximation, Flash, dual slope, ... all have pros and cons. A dual slope system will reject 50 Hz superbly if designed to do so but may fail miserably in a 60 Hz mains environment. RF interference (cellphones, mobile phones, pagers, wristwatches !!!, other instruments, ...) MAY cause issues. The ADC proper is part of this but the overall design needs to address the actual requirement. 

Potentially more may be said if better information on actual requirement becomes available. 

What country?