Monoprice used to be the only option for reasonably priced VESA mounts. Really, the sole option.
Now it's pretty easy to get comparable prices elsewhere if you look carefully for a sale. But if you need it now and don't want to look around, it's back to Monoprice.
Mounts come in three basic styles: flat, tilting and pivoting.
Flat mounts are very simple. There are two vertical bars that mount to the panel, and one or two horizontal bars that you mount to the wall. Notches on the vertical bars hook onto lips on the horizontal bar, and a retaining pin or bar keeps the panel from lifting up and away if bumped. This design trivially handles all of the VESA mounting options (50/75/100/200cm in each dimension).
Flat mounts vary a bit in how close they get the panel to the wall. You can pay a bit more for thinner ones, but that might be pointless if your cables need the room to exit from the back.
Other than that, get one that isn't too wide for the panel, but wide enough to hit the studs on the wall.
Tilt mounts are usually similar to flat mounts, but need extra room from the wall for the tilt mechanism and have to be built significantly studier for the same weight. You'll probably end up with no tilt, so the extra gap from the wall might make it not worth the extra complexity and cost.
Pivoting mounts have much more design variability, and are much more expensive. They need to be sturdier to support the weight, and designed to avoid wobble.
