The Teensy 4.0 (and 4.1) uses an NXP MIMXRT1062-DVL6A, which is another order of magnitude faster than the MK20DX256VLH7. Its datasheet doesn't specify the slew rate or bandwidth of its DAC, so I can't say what to expect with it, but even if we assume an order of magnitude improvement in the slew rate, that's still easily within the 100 MHz scope's ability to see the edge clearly.
Put another way, the bandwidth of the output of your Teensy is almost certainly easily within the 100 MHz scope's ability to faithfully show anything you generate with it.
Thanks mate this is great advice. I appreciate your help going into detail with the slew rate. Yup I got the Teensy 4.0. I'm really going hardout on the CPU.
I'm using parallel-IO into a resistor ladder instead of using the built-in DACs because its been too hard to sync the DAC to digital IO.
Its way, way, waaaaaaaaaaay faster - at least on my SAM3XE on my Arduino DUE. Waaaaaaaaaaaaay faster.
Well, in that case, then the effective slew rate is going to be the result of a combination of the slew rate of your parallel I/O pins and the RC time constant of your resistor ladder. This is where you get into some tradeoffs. You can use lower value resistors in your ladder, but if you do so then your parallel lines will have to source more current, and there will be limits to that (not to mention that the load impedance might well affect the slew rate of the output pins).
Because you're moving to a Teensy 4.0, but your experience is with the Arduino Due, you may need to perform some experiments to see whether your approach is really worth going with, as opposed to using the internal DAC.
In any case, most microcontrollers I'm aware of don't even come close to the limits of a 100 MHz scope when it comes to seeing the timing of the signal edges on the I/O pins. This really shouldn't be much of a surprise. Extremely fast edges on the outputs start to demand a lot more work on the board layout and the load in order to avoid ringing. In essence, you start to come up against transmission line considerations if you attempt to drive the output too hard, and most microcontroller boards like the Teensy are oriented towards hobbyists using breadboards and other inexpensive prototyping mechanisms wherein things like transmission line impedance matching aren't being considered.
So the bottom line here is that I think you'll be quite well off with even an entry level 100 MHz scope for this.
At this point, I think it's going to be worth asking what your expected future use cases will be and what your budget is. Different scopes have different things that they're particularly good at, so it would be helpful to have a better idea of the overall use case set so that scopes that have those as their strengths can be pointed out.