I'll drop some points here:
I don't buy premade external drives. I buy an internal drive and buy my own external enclosure
, mostly for the options I get. For all intents and purposes, they're both the same thing anyway. On the plus side, you also have a makeshift SATA->USB dongle. Useful for diagnosing drives.
A drive with USB3.0 or eSATA will pretty much achieve near native performance. I'm not sure what the overhead is for an external device, but USB3.0 with overhead and eSATA practically give you native speeds. Heck I run a Minecraft server on my laptop off a USB3.0 external, the server has no problems running or keeping up. It's USB2.0 you should steer clear from.
My current enclosure is basically a metal box around the drive. Even under its worst loads it doesn't tip over 45C. Idle it floats around 30C-35C. Operating temperatures should be around 40C, but I'm not worried. The current drive in it is over 2 years old now and hasn't had any problems, knock on wood. Then again, the drive is a WD Green, it doesn't operate at high temperature. If you're really concerned about it though, enclosures with fans are available.
I'm sorry but this just made me laugh a little: "also shutting down the device is bad since you just pull the plug out of the wall." Shutting down the PC has the same effect. I'm pretty sure hard drives are parked any time you're not using them and if you configure Windows to make sure it treats it like a removable device, as long as you don't have I/O ops going on, you can yank it out of the wall, turn off the switch, whatever you want to do. Oh and most 3.5" enclosures have a switch... but I guess that's the same thing anyway. And besides that, your back up should be offline when you're not using it. Are power cycles stressful? Yes. But I've probably had more on my desktop's current HDD than I have with my external, and it hasn't choked yet.
An external drive, especially a USB3.0 one, has the ability to be used by any computer. What your friend is talking about, can't be used by any computer unless it has the rack to go with it. What if you want to transfer data to a friend's computer? A laptop? Sorry, no interface to it.
One more thing. You can do interesting things with external enclosures. For instance, I put an SSD in my laptop, but stuff I want on it would exceed the capacity. So I put a condensed version of what I want on there, put the rest on a 2.5" USB 3.0 enclosure. There's only one wire and I use it when needed. Best of all, no real performance hit if I want to run directly off it.