FF Buying Options

When I click the Buy tab I come to 3 options:
- FF2 Software itself (electronic download)
- Backup CD (physical)
- Filter Library Backup (electronic download)

I don't have the money to pay for all the 3 options, so I am planning to buy only the software by electronic download.

Backup CD
I understand this is a backup of the software itself (correct me if I am wrong). So if I had downloaded the software electronically and I have proper backup of the downloaded copy, I would not need the paid Backup CD; am I right?

Filter Library Backup
I understand this to be a copy of all the filters available in the library at the time of the purchase; am I right?
- If I don't pay for this at the time of purchase, will I be able to buy the download later when I can afford to?
- If I don't ever pay for this, how would it affect my usage of FF2? If my system screwed up and I have to reinstall FF2 does it mean I have to re-download all the filters that I had downloaded before if I don't have the Filter Library Backup?
- Is there any way I can backup on my own all the filters that I had downloaded?

Sorry I had to ask so many questions before purchase. I am really confused by whatever the Filter Library Backup is for and how do I manage the filters that I have downloaded.