Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
No, it's not required; but it can't be a friction-belt drive (like a V-belt) - they will slip a little; putting a large amount of force on the tap. Gear drive works if the backlash is low enough and you can setup the control to handle it; but timing belt drive works best if it's not directly coupled. It helps if the pulley/gear ratio is an integer multiple; but depending on the control may not be a strict requirement.