hardcastonly

re-used powder should be fine(be damn sure you know what it is and don't mix two types)
primers left in cases, if you don't remove them and just resize the case ,should be fine
the neck size vs need to full length resize cases varies with the rifle, but yeah,
most times you'll find full length resizing may be required
I generally use cases lubed with a very light coat of Vaseline on the outer case as case lube
obviously you don't want primers soaked with lube
an ink pad with Vaseline , you roll the cases on, lightly works