You're on the right path with a mosfet switch- since you're switching DC at 30 something volts. The parts you mentioned are fine for the application, logic rated and 35 milliohms on resistance. That's about 0.3w dissipation at 3 amps so a small heat sink should be fine as well. You should have a fuse (poly or thermal) inline with the LED since 100W is enough to heat things to dangerous levels, given the right circumstances.
Your original post prompts me to say that you can kill yourself or burn the house down just as easily with a UL listed relay as a $1 bargain relay. When it comes to load switching, it's more about specifying the most appropriate device, with the required ratings, proper protective devices and installed in a proper enclosure with the correct wire size and insulation. Easy, right?
The power switch tail meets a market demand for makers that want to build and sell things that control AC mains without having to go through the incredibly costly product testing required to market a consumer product. That's how it survives at what seems to be rather high prices for what it does. One competitor and *poof* their margins would disappear.