Cases come in 2 flavours, Powered and Unpowered, if it powered it doesn't nessesarily take up Rack space, if its unpowered then you'll need to add either Case-Power or a Power Module.
The TipTop uZeus Happy Ending kit is by far the cheapest way to get a Case with power and start building a rack, I also find the uZeus power to be pretty resilliant.
From looking at Synth Shops I figured out what I needed to start with, I tend to read a lot about the subjects I'm interested in and only when I fall short do I ask questions of others who may know more than me. (This is how I do life and business, not just modular)
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.