I just bought a 2.2kw spindle and it arrived today I fitted it into the spindle mount (80mm you would need to buy new spindle mount and modify your machine to fit) and then weighed them both and it weighed 6.5kg's I would think that a 1.5kw would be far too heavy for your machine.

It's making me rethink my gantry and I have a lot chunkier aluminium than yours has in the first place.

I would go with the 500w spindle linked earlier if I were you as it seems more in range what your machine can handle and a drop in replacement.