I have no comment on the battery as all the third party ones I have ever bought went back because they were crap. You can never go wrong with Crucial memory, and 8GB will take care of you for a while. 120GB, IMO, isn't enough for an SSD, but thats only because I do video editing and I need to have plenty for scratch space. Samsung SSDs are pretty good, but see what deals Crucial has on their MX100 SSDs. They are usually not much more and I'm more apt to trust Crucial since they have been making memory chips for decades.
As for why people say only put the app folder on the SSD, its because back in the day SSDs were expensive as hell (I paid $300-$400 for my first 128GB SSD from Apple in 2011). The trick was to create a symbolic link to your home folder and store it on a spinning drive connected to the system. Since SSDs are much cheaper now then they were, its really best to put all your data on them. They are less susceptible to damage then HDDs and are much faster. What I would do is go into About This Mac and see how much storage you are using on your HDD in total and buy your SSD accordingly.
Typically, I recommend putting the SSD in main HDD space next to the battery for speed, but it wouldn't make a difference in this machine since both ports are SATA II. I typically recommend and buy the OWC Data Doubler. I have used them in multiple machines and have yet to have one fail on me. It includes all the hardware and instructions you will need.
Click here to view it on OWC's website.
Lastly, check out
B&H Photo's pricing. They are typically the same or cheaper then Amazon, but have free shipping and don't charge tax to most of the US (the only exception is New York state last I checked).
Good luck! I'll try to help as much as I can, just quote this post so I will get alerted.