I myself use to use Guitar Rig 5 along with Pod Farm 2.0 and neither of them satisfied me fully. I heard great things from them by other people, but I myself never got a decent tone out of them -- mind you I was still a beginner when I used them, so I'm certain now I could get a decent tone from them. I upgraded to the Axe-FX II XL and loved it the moment I got it out of the box. There is a learning curve, and some will argue most of its functions are pointless, but it has everything you could ever need: Flangers, delays, reverbs, amps, cabs and countless more. Plus, you get free upgrades for life, and these upgrades are always adding in new effects and improving the impulse responses of the built in cabinets.
Plus, if you like to tweak your sound precisely, this machine is perfect for you. Every single knob you can ever imagine is in every single amp, cab and effect. It is a tweaker's dream come true. Also, one feature that made me buy the Axe-FX especially was being able to tone match. That feature alone has made my tones pop out. Plus, if you really like the sound of a certain microphone setting, you can capture it and never have to use the real amp again!
But all of that is if you want to record. If you are going to be playing live, then I suggest you simply invest in a good-tube amplifier and a decent cabinet. Heck, if you have the space and option, just record using a real amplifier and cabinet; nothing is ever going to replace the natural sound of those.
Guitar Rig and Amplitube are powerful, and they can pump out decent tones, but the Axe-FX II is a professional machine that will make amazing tones out of the box. As I said, Guitar Rig and Amplitube are fine (especially if you are trying to save money) but if you want to make the investment, get an Axe-FX. I never tried the Kemper Profiling Unit, so I can't recommend that, but do research and figure out if either of those units are for you.
Computer wise, you sound like you have everything in check. I ran Guitar Rig 5, Amplitube 3 and Pod Farm 2.0 on my old gaming PC that had 8GB of RAM and a middle class processor and I never experienced any issues. If you have the option, upgrade to a better PC, but you should be fine for now.
As for what you should get, definitely get studio monitors, especially if you are recording. A PA system is good for a live environment, but studio monitors will allow you to make great mixes. I myself recommend the Rokit 6 G3 studio monitors since it's what I use, but there are plenty of options out there. Also, if you are getting monitors, you might also want to invest in a desktop computer unless you have a place to set your laptop on and keeping it stationary.
But as for the amp sims, I suggest listening to tones people made and reading all of the features and differences between the all. If you want to stay on a low budget, Guitar Rig 5 should be a fine option. I played with Pod Farm, but never really liked it -- Amplitube was much the same. But if you want to spend a bit of money, go with the Axe-FX or Kemper.
That was quite a bit. I hope it helps.