front speakers are good. buy components for rear speakers as the tweeter sound of stock is not that clear as anybody want it to be. and you can put that tweeters wherever you want.
Amplifiers are costly and one time investment.so buy more powerful amplifier once , so you can connect more things later to the amp if u want.
R300.4 will not do justice to your sub. and that sub you quoted can actually handle more then 150 rms but on rockford fostage website all speakers and subs are underated then the actual that they can handle.
R-400-4d or p500-4 would be better .you can go for jblGTO or kicker. they are good too.
And if you are looking for proper bass from sub , try any other better sub.
One more thing , amplifier output rms must be little bit more then the rms power of speakers or sub to eliminate distortion.
Good luck. btw
if you have verna fluidic never saw pic of your car on forum.
introduce your car and join our club. we will always be there to help each other.