Well of what i know and researched there is no simulator that lets you put your own parts in etc but on youtube there are Thousands of builds and they are very clear and people know what they are talking about also it's not a simualtor but you can go on websites like https://www.vibox.co.uk/http://www.cyberpowerpc.com/ ,etc just to check comp ability and the full cost but the website you used is fine if you want to build your own pc this is a good youtube chanel showing you how to build and tells you all you need http://www.youtube.com/watch?v=lPIXAtNGGCw and then just go to part 2 etc it's really good and if you dont like it just go on other videos and here is a list what you would need to know
1) Start with the processor you want.
2) Decide what kind of graphics solution you want.
3) Pick a motherboard that is compatible with cpu, supports the number of GPUs you want and has any features you may want.
4) Get system memory that is listed on your motherboard compatibility list.
5) Get a case that will fit your motherboard and any peripherals and storage you want (If you're getting a high performance graphics card ensure that your case is long enough).
6) Pick a high quality power supply sufficient to power your build and ensure that you have a decent cpu cooler and enough fans to ensure air flow through your case.
7) Don't forget misc extras such as thermal paste for the cpu, brackets if you need them for SSDs, connectors and cable ties.
I found out that the fx 6300 comes with a fan with thermal paste on it. Is it still necessary to buy thermal paste?
As far as I know (AFAIK), all retail processors come with heat sink/fan assemblies and pre-applied thermal paste. Technically, no. You don't need to purchase thermal paste, but it is still a good idea to have some around just in case. Once the heat sink touches the CPU (and thermal paste makes contact), you cannot remove the heat sink without re-applying new thermal paste.