It is very dangerous to expire images with bpexpdate -m media_id ... You can throw away images that should be kept. There is no guaranty, that no other images are on those tapes (Multiplexing, later, earlier starttimes, etc. pp.).
Best way is to use...
Diskspace depends very much on the amount of jobs and the retention of the data in OpsCenter Database.
Also I would recommend at least a quad core machine AND 16GB RAM. more ram is better and more CPU Power is also better for getting reports.
For D...
If you mean the dashboard
No there is no way, and it is also not good to do that. You should care about your status code 1 jobs and eliminate at best all of them.