Pressure Reducing Valve
30-40 is about minimal. 70-90 is considered nominal. 120+ is high, and most fixtures - as well as your heater PRV - are rated at 150. If you have an expansion tank on hot water system, even 120psi is more or less ok though not desired. You 105 is totally fine.
I would put in a pressure gauge with a lazy arm, leave it there wor a couple weeks, and see what was highest pressure recorded. If 105 is tops, don't bother. If it goes over 110, you might want to install the reducer.
The proper installation should be described in the reducer valve dicumentation. I.E. Installation sheets for Honeywell valves specify horizontal installation only.
Far as how long - these things tend to last awhile, especially high quality ones. Though water chem, input pressure, etc affect longevity. And breakage can be varied - it could simply lose effectiveness and stop reducing pressure (happened to a friend of mine), or it could start leaking - just like about any other valve.