I am not especially keen on electric showers. They cannot deliver as much hot water as a combi or a cylinder, but can be a useful standby in case of boiler breakdown.
Most often one of the heating elements burns out with age, so you lose either low or medium heat, and high is not so high.
Electric shock suggests a wiring fault, which should be looked at by an electrician. They are made so that a water leak inside the casing will run out of a drain hole. Power must be isolated at the CU before attempting repair or inspection, and the circuit tested for dead.
You need to look at the power available. The circuit breaker will be marked "B32" or "B40" or something similar, and that will limit the amps on the circuit. You can't increase it unless you run a new cable for the circuit, the breaker is (should be) matched to the cable size that has been used. All circuits in a bathroom should now be RCD protected. If yours isn't, and the consumer unit (fusebox) is fairly modern that can be fixed by an electrician, probably fitting an RCBO which might be quite easy, but is not an amateur job. The pipework should be bonded to earth in most circumstances, which, again, gives some protection against shocks.
For the easiest fit, the same brand used again will usually fit the existing positions of pipe and cable.
If you have either a combi or a cylinder, you can probably get about three times as much hot water by changing shower type.