You could go whole hog and do the AWD system, but obviously time and money come into the equasion.
Can i ask, is it a transverse 4 or a horizontal 4 engine? if it's a horizontal, then things become expensive and tricky very quickly. Also take into account that, if you did do it and you want to sell it. how many people would be interested? and then there is insurance to sort out too. so obviously the cost doesn't just end after the cars completion...