As I understood the question you posed, it's not whether more equipment is useful, it's what would be a good investment now and what do you absolutely need--right?
I have had multiple scopes (including the Snap-on Counselor way back in the 80's) and many scan tools over the years. I still have about 10 oscilloscopes lurking about. I could have had more, I could have had less, I suppose. Sometimes they make a job easier, sometimes (as is claimed here) they make it possible. I also had a complete machine shop, including a crankshaft grinder--that's $100K alone--and you can't regrind a crankshaft without one, right? I'm all for tools, but as a devil's advocate and adviser to one on a limited budget, I'd point out that you can just buy crankshafts if need be, and in the case of the Honda 3.5L (the second video) I'd point out that a) he never actually determined the exact cause of the failure and b) he could have solved the problem by simply insisting on redoing the previous repairs with all-OEM parts. I'm not criticizing his approach in any way, I'm just questioning whether you are ready to go down that path--or if you need to. You said this was a bread-and-butter issue, and I can tell you that while I've done my share of "superhero" repairs, fixing the ones that nobody else could, that isn't the path to riches or even business success. You can learn from these adventures, but I'd be cautious getting into someone else's messes. I certainly respect you for not wanting to be a parts-thrower, but don't get sucked into cleaning up after them after they've emptied the customer's wallet. I don't know your local clientele, but I did know someone that lived and worked on one of those islands that told me that customers were generally pretty cheap and didn't appreciate large repair bills.
To do this particular type of diagnosis, you really need a PC-based unit and the Picoscope auto-specific ones are the best. If you already had something like a Fluke 98, a basic Picoscope would more than suffice, as their feature set doesn't overlap that much. The Fluke would not be good for this type of work because of it's limited number of points on the screen. I did it before the PC based scopes came out, but it's tricky. You might also look at other low-cost PC based units--I think Hantek has some pretty cheap models. You can never have too many channels, though.