My current Installation of DCS (current open beta via Steam), containing
Modules: A-10C, F-16C, F/A-18C, FC3, UH-1H
Terrains: PG, NTTR, Normandy
uses 165 GB of disk storage. I would recommend a drive of at least 512 GB, maybe better 1 TB for future.
If you want to get the whole DLC for DCS you'll need primarily a lot of money ;)