My prior probability that any given household appliance or fixture will break and/or need maintenance in a given month is on the order of 5%, obviously with some variability depending on what appliance we're talking about.

There's your problem right there. Note, that this prior effectively assigns zero probability to the "prior owner didn't maintain the house" hypothesis.

What you should have done is assign some (non-zero) probability to that hypothesis, then when something breaks, one would updates towards the "poor maintenance" hypothesis.