My experience was pretty much the same as GMack described. I'm also a non-current private pilot and found the test to be quite similar to parts of the the Private exam. I spent about 10-12 hours reviewing/studying, in part because I wanted to ace the test.
You don't need to buy an online course - everything you need to know is downloadable on the FAA website.
That said, there is a lot to learn. Quite a bit of it is stuff that doesn't really apply to Phantom-size quadcopters, but remember that a 107 Remote Pilot certificate allows you to fly rotary and fixed-wing unmanned aircraft up to 55 lbs.
Buy a paper sectional chart and make sure you know what every single mark on it means. Make sure you can determine what class of airspace exists at all altitudes at any location. You should know whether you are allowed to fly at any location and altitude. I am not exaggerating here.
Learn to decode a METAR and TAF completely. If you don't know what those are, don't waste your money on the test - you won't pass without that knowledge.
Know all of the part 107 operational regulations, retesting schedule, accident reporting requirements, etc.
Know radio communications phraseology.
FAA is big on aeronautical decision making, risk management, crew resource management and hazardous personalities/attitudes. Look up the FAA publications on that stuff and know it.
Download the testing supplement and become familiar with what is in it. You can find answers to some questions in the material contained in the supplement. (For example, there is an extensive legend of all the symbols on a sectional. Forgot what a dashed magenta or dashed blue line around an airport means? That info is in there, but it won't do you any good if you aren't aware of it.)
If you know all the above stuff cold, you can relax and know that you'll do very well on the test. If you skip learning any of the above subjects, you might not pass unless you nail everything else. If you don't know the sectionals, METARs and TAFs, you definitely won't pass.
Good luck. Let us kknow how you did.