Roof Needs Replacement - Seeking Advice (Saint Paul, MN)

My wife and I recently purchased our first ever SFR. We've been told by contractors who've visited our property that we should look into getting our roof replaced within the next year or two.

As investors, what caused you to pull the trigger on getting your roof replaced? What degradation signs did you look for? How did you fund the replacement? How did you go about hiring and trusting roofers to get the job done? 

We appreciate any and all roof replacement guides and rules-of-thumb you can provide, thank you!