With great respect, you are talking to the undisputed King of budget reefing right here lol!
And I do agree that the hobby can be done *relatively* inexpensively, but I can't call this an inexpensive hobby, if that makes sense.
One thing I want to mention about the light you're considering is that I know from having bought very similar, inexpensive lights for some freshwater tanks in the past, when it says 18"-24", the light itself is only 18" long. The little legs on the ends extended outward so that the light can sit on a tank as long as 24", but you will have a few inches on either end of this light that won't really be fully lit, and may look weird.
I'm also just kind of skeptical of the price and generic brand name, as others have said.
I thought about buying your IM 20 when I started my IM 15, and one thing is that with the 24" length your tank would be best lit by two small pendant-style lights, or a true 24" fixture, in my opinion. I think this is best to avoid dead spots and shadowing, even if you only want to keep lower light corals.
I have never used them, but I have heard a lot of reefers here talking about Noopsyche lights, if I'm spelling that right. I also believe that the Hyger brand name has a following here, and is available on Amazon.
You could also consider some old-school solutions. For example, I think a couple PAR-styled LED bulbs would work on your tank, you'd just need some cheap fixtures (Goodwill, Dollar Store, your local Mega-Lo-Mart, etc.), something like these:
Amazon product ASIN B01M0PF72P
Or, for simplicity you could look into a 24" T5 unit.
I hope that helps, good luck with your tank!