you dont actually need coilovers to drift. but yes it would help alot. if you dont have the funds for them, im sure you could find some decent stiff springs and shocks. suzuki sierra front shocks bolt into the rear of a ke70 with no mods at all and there stiff as anything, tho there super short so your car would have to be pretty low.
maybe uprgrade your fronts to 86 struts. making your choice of spring and shocks abit more variety, Strut Braces will also help stiffen up ya car.
if you wana go spend ya money on coilovers. AJPS make custom front coilovers for ke70's. the about $950 for the fronts. you couldnt put coilovers in the rear without some modifications and extra bracing...
(6kg rear and 8kg front is a good combo)
next is your diff. if ya dont want to fork out $1000 + on a diff. (T-Series with a 2way LSD) best option would be just to weld it up, as there is no such LSD for a ke70/ae71 diff (tho i heard you can buy full spools from repco) I have a locker in my Ae71 and it serves me fine. its just not road legal.

I demand pics of this ke70 your buying aswell