i have a capri rear axle which i believe doesn't have lsd, would be able to fit a sierra diff or another diff to give it lsd

Nope. You could get one out of a 2.8i Crapi though.

I'm thinkink about using the 2l 16v vaux engine will i need lsd or will i get away with out it?

You dont need an LSD, but it does help get the power down, especially from standing starts or accelerating out of a corner. And you will be able to do proper donuts...the real reason for an LSD

that's persuaded me, i'm gonna start looking for a capri 2.8i axle