I wouldn't really worry about it.
I've either owned or known of literally dozens of Macs over the last couple of decades, and the number of power supplies being replaced is zero.
The PS in the Mac is designed to be the right capacity for the Mac in question, plus some overhead, and it's of decent quality.
What I see on the PC side are people who make their own boxes that underestimate the current draw and install a less expensive PS than needed, and over time that PS tends to fail. Or, they went for a cheap PS and it dies.
This also happens when folks install a bigger graphics card and forget about the extra load there as well.
If the PS fails on your Mac in the first year (again, unlikely), simply have it fixed under warranty. Applecare can be bought to extend the warranty past the normal expiration, but nicely enough, you don't have to buy Applecare right away, but you can wait the entire original warranty period to make up your mind.