Yes, contacting the borrower prior to owning the note would violate most any seller's NDA, as well as a number of regulations which protect borrower privacy. This is a "bright line" kind of thing which we just don't cross.
Regarding paying taxes and municipal liens, there's no hard and fast rule here. One thing to keep in mind is that delinquent taxes have significant penalties attached, so if you believe you will end up with the deed, it's generally best to pay them now rather than later to avoid additional accumulation of penalties. That said, I have left taxes unpaid in foreclosure situations where the property is desirable and in a relatively active RE market, i.e., when I have good reason to believe that a bidder at the foreclosure sale will pick up the property and take on the tax bill. You might ask your attorney about the foreclosure rules related to taxes are in the county. You may or may not have to pay them prior to the sale.