How to swap out old set of RAID5 hard drives to new unit

[chuquy]

Caveat: I'm not tech savvy and have no plans or time to be. Please go easy on me.

After eight years, the 4-bay Mediasonic eSATA, USB 2.0, Firewire 400/800 - HFR2-S3B enclosure I used for my multimedia server appears to have died. It seems to be an electric/power problem. All of the 3TB HDDs were fine. One day, the fan wouldn't work and then it wouldn't power on. I took out the HDDs and it still wouldn't power on.

This was my setup:
4-bay enclosure (Mediasonic) with 3TB WD HDDs in each bay, in a RAID 5 Array. I also have a separate HDD Dock with another 3TB HDD (it still works fine). I attached both via USB to an iMac, which runs Plex server. I access my media using the Plex app on an Apple TV over wifi. My Apple TV is connected to an old Sony HDTV.

My first priority is to protect the data on the HDDs. My second priority is to upgrade my setup. I have questions about achieving both priorities.

Priority 1 Questions:
1. How do I swap out the old 3TB WD HDDs into a new Mediasonic 4-bay enclosure? Is it possible, and if so, what are the steps? Would I need to set up the new enclosure in RAID5 before plugging in the HDDs? Anything else? Or, when I plugin my HDDs, do I run the risk of them being reformatted? I don't want to run that risk?
2. Would it be safer to first make backups/duplicates of each HDD? I have a two-slot HDD reader that can copy automatically.

Goal 2 Questions:
1. If I can simply buy a new 4-bay enclosure and place my old HDDs into a new enclosure, what model do you recommend I buy? I was pleased with my previous setup and it lasted more than eight years. I don't, however, want to rely on streaming if possible. Even when I had this set up while living in the U.S., I experienced buffer delays. In Cambodia where I'm currently assigned it's worse. I would prefer my media physically attached to my TV. Does Mediasonic have such a system? I also want to be able to write from my iMac to my disks faster. My old system took too long. I'd also like to improve my AV quality.
2. Is RAID5 best for my needs? I've had an HDD fail in RAID5 and it was frightening. I have about 9TBs of data in my current RAID5 config.

VERY much appreciated.

Eric