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AE only uses the GPU for a few things. The big one (the ray-traced renderer) is considered an obsolete feature, so I wouldn't worry about it. Unless you're using the ray-traced renderer to create 3d geometry (instead of the much more powerful options we now have), you should just set your compositions to use the standard renderer.
So, unless you're using a third-party effect that actually uses the GPU (like Element 3d, Red Giant Universe, etc.), the GPU is not a concern for AE use (and, if you are using one of those plugins, whether or not you can enable that switch to use the GPU to accelerate the ray-traced renderer is irrelevant).
Therefore, it seems that the issues your are experiencing are unrelated to the GPU.
From your description, it sounds like you're bumping up against the main known issue with El Capitan. Long story short, dock any panel underneath your timeline and your playback of cached previews should be real time.
See this example (the render queue is docked below the timeline panel):
If you would like a further discussion on the matter (and some other suggested workarounds), see Todd Kopriva's post in this thread. (I've linked directly to it, so it'll be at the very top of your screen if you click it.)
This should make your computer work with AE just fine. If not, there may be other issues we can sort through so you don't have to buy a new machine.
My question is, should i go with the 5K iMac, the entry level Mac Pro or a custom build PC??
So, to answer this question, a PC. It seems to me that Apple has put their focus on iPhones and iPads and have abandoned their professional user base. People were concerned this would happen when they stopped exhibiting at NAB some years ago and their fears have been proven correct with the Final Cut X debacle and the pro-unfriendly new Mac Pros. If you were just doing photography, an iMac may still a good choice (because you can't beat those screens and you don't need a ton of power), but if you are doing motion graphics or vfx work, PC is the way to go. You get much more bang for your buck.