IOS – Messages App in iOS simulator

1GB of RAM will kill you: frankly, I can't imagine that it would meaningfully run. I'm currently using a 2009 MacBook (not Pro) with 4GB of RAM, and for the first two or three hours Xcode runs pretty well.

After that, though, it gets slower and slower. On a pretty small project Xcode will be using multiple gigabytes of RAM with the OS swapping 2-3GB to disk. Before long I not only have to quit Xcode, I also have to restart the machine to get things back up to speed. (To head off potential commenters, no, it's not something else on my system, it's Xcode 4: no other piece of software, including Photoshop operating on a 20+ GB file, fails to clear up the swapping speed problem after quitting the app.)

I can safely say, both from my experience and from talking to others, that 2GB of RAM is the bare, bare minimum, and it's not going to be a good experience. I'll be replacing this system soon, but in the meantime I'm upgrading to 8GB because I'm losing too much time to only having 4.

At the moment, you can't really do this. Although I suspect that Apple is looking to integrate iOS and macOS in the future so that this sort of thing can be done (purely speculation).

GarageBand on the Mac is different from the iPad version (it's more comprehensive) and it has been available on macOS for far longer than on iOS.

It is possible to run iOS apps in the Simulator app that comes with Xcode. However, it's not as simple as downloading an app from the App Store. You'd have to get the source code for the app, compile it in Xcode and run in the Simulator. Most apps being closed source, this won’t be possible!