Excuse the real lack of knowledge here!

I have an old Aqualisa Shower which, after, leaking into the back
unit, I have isolated the water flow to. It's been like for the past 6
months now, has a few other things wrong with it (outer bits crcaked
etc) and I am currently debating whether to have it repaired or
replace with a completely new shower (without spending a fortune as I
don't intend to stay in the house for too long!)

My water system is gravity fed. Can a normal electric shower or power
shower, say, like the ones readily available from Triton (T80si/Ivory
II as two examples), be installed easy, or do these usually run off
the mains water supply? I worried about buying one of these and then
finding out, it'll be a costly job to install.

I have a pump in the airing cupbord - WILO SALMSON PUMPS Class F? -
next to the water tank. Is this pump boosting the water flow to the
shower or doing some other totally unrelated job! I ask because I've
read how you can install shower booster pumps to increase the flow
rate.

I really would appreciate any advise (just starting out on the
Plumbing learning curve!!!). Thanks!