im planning on building a kit car, id like to use an escort rs turbo engine (of a sort) with a sierra gearbox. I also have a mk1 escort estate donor car, for some of the parts but will the rear axel take the abuse from 200 horseys?

I don't really know anything but I'd imagine a MK1 axle was never designed with that kind of power in mind. You might be better off going for a sierra diff/de dion setup or using a bigger capri or cortina axle.
Then again it might be just fine.
Sorry to be of no help whatsoever

in such a light car, yup will be ok,

is that a for sure or will i be looking at a replacement within 15k or a few years? because if that happens i will be sourcing a stronger version from something along the lines of a rs2000 escort which would be quite a challenge when it would be easier to fit a sierra cosworth or xr rear whilst its being built?

nothing is for sure apart from meeting our maker,

personally i would happily run 200bhp thru the axle in a car weighing 500-600kgs, you wont have the grip to burst it.

The vauxhall viva axle in my sylva never ever broke even with a 2.0 zetec on Dellorto 45s and weber alpha running through it.
This axle originally had an 1159 pushrod vauxhall engine pumping about 45bhp into it.
As Mr Ison says - light weight and realtively low grip are your friends here

ideal
Im now a lot happier.