The 1060 recommends a 400 Watt power supply, but you are smart to not trust the OEM PSU to deliver on it's rating. the PSU is usually the first thing that get's skimped on for pre-built systems, so they can put the money towards components to use as system bullet points.
Really it's hard to go wrong with most reputable brands. Just from my own experience, I've purchased a number of Corsair and EVGA power supplies and have had no issues with them at all.
Some Examples:
EVGA 500+ BronzeCorsair CX500.
For a bit more you can also get a modular power supply. Very useful as it avoids having a bunch of wire spaghetti all over the case, since you only plug in the cords you need.
Another concern could be the length of the card. I found when upgrading from a 770GTX to a 1070GTX the 1070GTX was actually too long to fit; I had to remove one of my hard drive cages and re-arrange some of my internal disk drives to make space. For a XPS 8700, it looks like (based on some pics) the drive cages at the front might not designed to be removable which could complicate installation.