Well, I'm not going to tell you to waste money on a Mantis cab. I'm going to tell you to get one because it WON'T BE a waste of money, as opposed to building a 1-row system that lacks space for proper functionality unless you do it all in 2-4 hp modules...at which point the build will be unplayable unless you've got toothpicks for fingers. I know people see these all the time on YouTube and think that "hey...I can do that too!"...and the simple fact is that the people doing those clips have been working with modular for a long time and while THEY can set up a workable build in some tiny cases, those builds tend to be "mission-specific" for a certain piece or specific gigs. They almost never result in a good, all-around system...which is what the goal SHOULD be.
Or a far better idea might be for you to snag a B.2600. Don't worry, NOBODY likes Uli Behringer, so don't feel guilty. Fact is, the ARP 2600 was and still IS one of the best teaching synths in existence, period, full-stop! It also comes in at $600-ish, while your build above is considerably higher in cost. No, it won't be a waste of money either, as that 2600 plays just fine with Eurorack, and once you get some modular experience with it, you can use it as a "core" device in a larger Eurorack setup. And thanks to Behringer's slavish replication of the original, their 2600 acts and sounds just right...and this is coming from someone who first used an ARP 2600 back in 1980. Tack on a basic sequencer like the Korg SQ-1, and you're golden!