I do understand this is just one factor in evaluating investment properties, but is there a “rule” or general number for price to rent ratios that indicate a “buy”?  

Im right on the cusp of of being the oldest millenials or youngest gen x, so my answer might be influenced by my age.....I think once you are done with high school, you should be out the door and learning to stand on your own two feet.

There are numerous factors at play and your real estate tax professional is best positioned to opine on that.

Returns on cash invested will be huge when you factor in the appreciation gains.

What key factors should first-time borrowers consider when securing funding for their initial real estate investment?

Most contain a "fudge factor", where they say "We believe this represents this segment of the formula, and are often inflated in favor of the current administration, and later adjusted down.

If you anticipate a big change in interest rates, what factors do you believe will drive this change?

As much as 50% discount from replacement cost, so there is that "spring" built into values, just a matter of time until that factor pulls prices up, question is how long. 

With any lender though, the DSCR requirement is probably higher,  some lenders start asking for tax returns, in place rents becomes more important, current occupancy, and other factors that a residential property does not have to worry about. 

The underwriting is based primarily off of the property, not you as the borrower so the main factors evaluated are property value, DSCR ratio (cash flow), LTV, and FICO.Also, I am from Boston and know the area well.