It doesn't necessarily need to be an i5 or i7. Dolphin only uses 2 cores so even an i3 can be sufficient. Having more than 2 cores will allow background processing for other programs (that aren't dolphin related) which may help performance though.
The most important thing for dolphin is strong single thread performance which is why it should be an Intel CPU as pre Ryzen AMD CPUs have really poor single thread performance. GPU performance doesn't really matter as long it is sufficient and 4gb of RAM is plenty, which is like the standard minimum.
As long as you have a 4th gen or higher CPU you should be fine. I would say if you don't choose a laptop with a dedicated graphics card (and don't care about upscaling) intel HD 4600 is as low as you should go.
Another note, you may want to avoid CPUs that end with a U, which denotes ultra low voltage. The goal of these is to reduce power consumption to increase battery life, which may reduce performance in Dolphin. Although newer CPUs are powerful enough for this not to matter.
