In a nutshell: get as much as possible! :-)
As a rule of thumb, many users suggest (Number of cores) * 2GB. In general, more available RAM will always help caching and therefore increase your build speed.
Most builds require a couple of 10GB, so HDDs are the primary storage technology for OE-based projects. Most users should be fine with a standard drive and 50-100GB of free space, although working on multiple projects, automated or world builds can significantly increase the storage footprint.