Most parts can be printed without supports. The Middle Case has a few extremely small overhangs that DO require supports, unless you choose to print t...he version of the file that has tiny custom supports built in.
Print spring with 100% infill. Ideally, the filament path should run the length of the spring. This can be achieved by forcing the infill to be horizontal instead of diagonal or by setting the perimeter count to a high value.
Case parts should click together with a little force. If they feel too loose, a small drop of hot glue will be more than enough to keep them stuck together.