AE will never whizz through all RAM previews. That's an illusion not worth hanging on to. Check the various discussions on MP rendering and performance optimizations. There may always be conditions that prevent maxing out resources like specific CoDecs and effects only working in a single thread and so on. the rest is too vague. Speed and performance are influenced by a million factors. You could follow Adobe's recommendations and buy the best and fasted SSDs for storage and caches, but achieve nothing because you don't do the kind of work that this recommendation is based on (combined workflows with Premiere and massive footage I/O). In the end we can spend the next three days discussing this, but the simple truth is that you have to know what you need and a balanced system is much more important than having one standout high-performance component like a super-expensive graphics card, but it being hindered by less effective and efficient components. Get whatever current core7i is in your price range, 32GB of RAM, 2 SSDs for system and cache and 2 or 3 platter drives for storage. Beyond that there is really not much to tell.
Firstly, sorry for the late reply but I've been doing some research and I wanted to ask your opinion between AMD and intel processors. You recommended i7, correct? So I was wondering if you leaned more towards intel, if so, why?
Also what is your PC specs, if you don't mind me asking