You can almost certainly get ZX Spectrum emulators for Android or IOS, but the problem there is how to connect a proper keyboard (maybe via bluetooth?) or a switched joystick, without which some games would just be unplayable.
Probably better to use something which outputs to a decent sized display, I think, especially with most of us now having middle-aged eyesight.
The Raspberry Pi 3 is just about usable as a basic home computer, including for Internet surfing and locating / downloading software to run on your emulator - the Pi 4 ought to be better still for that purpose but it is sufficiently different to the earlier Pis that a few things still need tweaking specifically to run on it, including, at this time, Retropie.