Help developing backup scheme for business - two 500 GB hard drives in machines

If you have no space for two backups, it's possible to create backup using custom scheme setting clean-up rules to 'when there is insufficient space in the target'. It's a little bit dangerous, though - if something goes wrong when old backup is already deleted, but the new one is not done, you are left with no backups at all.
Dangers of incremental backups are exaggerated, if you have reasonable chain lenght (say, 20 incrementals) and validate them. Differentials backups are independent (of each other) and get useless only if the full is corrupted.

I'm planning on setting up each system to alert me via email if the backup fails. If that's the case, would the 1 full backup solution be ok if I were notified and kicked off a new one the next day?

As far as differentials are concerned, would it be better to set up a schedule to do differential every night? How does that work when you run out of space on the drive since the differentials are all connected to the full?

I thought about doing incrementals and telling it to clean them when either it runs out of space or every 30 days, but I'm terrified that even if I flag validation, I'll go back and have to recover something for the president of the company and have to tell him that I couldn't get it because one of the incrementals somehow got corrupted.

If would be OK unless the main drive crashes the same day.

If you use this cleanup scheme, all differentials are deleted when that base full is deleted. But having differentials in addition to full allows to keep more 'restore points' to roll back to, not a single backup.

If you do the full backup each 30 days without incrementals, the file that you need to recover may be created just after the full was done, and lost during these 30 days. And you won't be able to recover it just as if it were corrupted inremental. So for important data (where data loss implies not just a day lost reinstalling windows and setting up working tools) full backup to the disk in the same PC, which can contain only one copy and can be destroyed along with the main drive, is not enough.

So what would you set up in my case using the two 500 GB drives?

Weekly full, daily differentials for whole disk, cleanup by 'when it's insufficient space on target'. And separate file backup with 5-6 level TOH for important files to some storage not on the same PC.