Hmm, Maybe I can stick some zombies together by the base, paint them ghostly, that might do.
Any spare models you have hanging around from any other army or fantasy range would work - random goblins, Empire soldiers, Skaven ... anything. It's the paint job that will make them the spirit hosts. Even a random knight of some flavor thrown in the mix. Hmmm ... I think I just made me some new spirit hosts!
45 Zombies is a good number, but not as the unit starting size. Start out at 20 and aim to grow them both to 40-45 each, you can also start to raise some smaller units during the game so it is always worth having plenty of zombies laying around.
as Karnal says you do need some cheap drops, the multiple zombies and/or spare models is a good idea for spirit host
Yeah, that could work. I'm not sure how picky you are on the aesthetics of how things look. Figure out how to connect them and then fill the gaps on the edges (basing should fill in the top nicely). Boom! 40mm base!