Converting car to battery power now seems to involve new high-power and very expensive AC traction motors with all the highly expensive control gear that entails, along with the highly expensive Lithium Ion battery packs needed to feed it.
Thus £20k is a cheap job, many companies charging 2 or 3 times that if you want to pay for it to be done for you.
Fine if it is a £200k Ferrari or such like and want the latest hi-tech solution.
A few years ago you would use a DC motor, along with the much cheaper control gear needed and a boot full of lead-acid batteries.
Set the DC motor to feed into the original gearbox, nice cheap solution.