The very basic reason LEP stopped going to higher energies ( it reached over 200GeV center of mass, at the last stage, LEPII) and the tunnel was used for LHC is synchrotron radiation .
Note that radiated power is proportional to 1/m^4
It is not possible to feed a circular beam of electrons the energy needed to raise it to higher energies at the radius of LEP, it is a loosing game, The energy would go into feeding synchrotron radiation. The reason the same radius can be used for much higher energy protons is the ratio of the masses of electron to proton.
Synchrotron radiation is not present in linear colliders and that is why the next electron positron accelerator will be a linear collider the ILC.
What would happen if the scientists would use leptons instead of hadrons?
It has happened at LEP and LEPII with electrons on positrons. If the scattering is not elastic a lot of hadrons appear, as well as leptons and Z bosons. The data from LEP confirmed the calculations of the standard model for elementary particles to great accuracy.
Especially: What would happen if they would collide electrons?
from the previous paragraph, the standard model predicts what would happen if electrons were scattered on electrons : all the variants of the feynman diagram possibilities would appear also.