Use it mostly to play games. I suppose a ide sidecar hard drive is a better choice than scsi? Does the scsi hard drive have to have parity to be used with the side car hard drives such as the Supra 500XP? Looks like you get the added benefit of extra memory when you have one of these sidecar hard drives. Is there a limit on how large of a hard drive you can use with a Amiga 500?
Lots of questions

1: SCSI vs IDE does not really matter. It mostly depends on whats available. But yea, sidecars are the preferred way. However. there are some interesting DIY internal options if your steady with a solder iron and are able to source the necessary parts.
2: Drives in amiga
does not have any form of parity by default. It is up to the user to perform regular backups.
However. there has been some success with raid controllers in amigas before. Try searching up the topic on the forum.
4: Some of the sidecars do provide optional ram sockets in various forms. But rarity increases as well.
5: YES. by default (using FFS=Fast File System) max is 4GB total size on any given drive. And you should never exceed 500MB for boot partition or 2GB size for other various partitions. Other rules apply when using different filesystems. But be aware of compatibility with your current kickstart....!
When you get familiar with the systems and are able to experiment, you can increase HDD sizes. Quite a lot. But the absolute max DRIVE is around 128GB (if i am not misstaken)