A lot of your questions are subject to personal opinions. There is a lot of objective testing of mobos, RAM, CPU coolers, etc. online if you desire factual information vs. opinion.
I will give you my opinion based on personal experience, a lot of testing and some online objective scientific testing.
The Seasonic PSUs as you probably know are top shelf so it's just a matter of picking one that has sufficient power for your application. I would recommend the 550w unless you have a very power hungry GPU card, which could require a >600+w unit. If electricity is expensive where you live then a certified "gold" model may save you money long term. If not any of the Seasonic models of 550w or higher will be fine.
Asus, Asrock, Gigabyte, MSI and others make good mobos IME. I would not pay a premium for an Asus or Gigabyte mobo however as they have not proven to perform any better than an Asrock, which IMO is the best value in AM3+ mobos right now. The Extreme9 Asrock has a proper VRM circuit to handle the high power consumption of the FX-8350. If you decide on a Gigabyte mobo be sure to get one of the new "XFA", "UD" models as the prior models can NOT properly supply sufficient current to the FX-8xxx series CPUs without overheating the VRM circuit. The mobo VRM circuit is very important when using an FX-8xxx CPU so you will need one of the highend 8-12 phase VRM circuit AM3+ mobos to handle this CPU.
Any of the name brand DDR3 RAM will work fine in your PC be it AMD, Corsair, G.Skill, Mushkin or the other brands. What is important is to buy ONE RAM kit of the total RAM quantity that you desire. In other words buy (1) 8GB kit not several RAM kits to total 8GB. As far as RAM frequency 1866 MHz. is the default FX RAM frequency so that would be fine. You don't need top mounted heatsinks on the RAM so don't pay extra for them as DDR3 RAM runs cool.
As far as HSF coolers there are many good ones. The link below shows an excellent database of thermal efficiency and capacity of many of the popular HSFs tested under controlled scientific conditions for accurate results. I am using the Aegir SD128264 on my FX-8350 OC'd to 4.7 GHz. and it works wonderfully and is priced right, IMO. If you chose a tower cooler make sure that it will fit in your case and clear the RAM you chose.