get a heavy duty A15 vannette pressure plate, they have the highest clamping load of all the A series clutches. and if you actually get the clutch from a clutch specialist you can get them to have it made with an even higher clamping force than any of their off the shelf clutches.
for a heavy duty a15 clutch kit with just an organic clutch plate it should be under $200
then a solid brass button centre would be another $100-150 im pretty sure
if you go a cushioned button clutch the price goes up a bit more, and all its doing is giving your gearbox and diff a little easier time if you change gears hard.
from personal experience it is no harder to drive a car with a button than it is with a cushioned button. ive found that the clamping load of the pressure plate has a lot more to do with it as it tends to throw your foot off the clutch pedal when you are releasing it

i wouldnt go any of their off the shelf kits that are to suit an a12, as they would probably have the same clamping force as a stock a15 clutch

if i were you i'd just get an uprated A15 heavy duty clutch with an organic plate. my brothers corolla was running a similar thing(uprated celica HD organic) behind a turbo 3t and i cant remember it ever slipping until the gearbox front seal leaked and got oil all over it. and that didnt get driven nicely

even the sports tough HD clutch i got for my L series was only $200 and then an extra $100 for a solid brass button centre. and i know A series stuff is cheaper

EDIT, i forget if the A12 ans A15 clutches are the same size as my experience was in getting a clutch for an A15. but even so it would possibly be worth changing to an A15 flywheel so you can run a larger clutch if that is the case