Batteries are easy to replace, just buy one on ebay/amazon from a well-reviewed seller that advertises compatibility.
You should be able to use standard sodimm RAM. If I remember correctly, I was able to get 8GB running at 1333 MHz just fine on my 2010 pro. You should be able to install up to 16GB even if it is not supported, but you may want to research compatibility first. Stick with the lower voltage RAM and only used a matched set (don't just stick a single 8GB stick in)
You can use any standard SSD. I would recommend a samsung 840 PRO or 850. I actually took my superdrive out (I hardly ever used it) and installed a HDD instead, giving me both SSD and HDD storage. If your new drive via usb, you can clone your current drive using a program called "superduper." It works pretty well. After cloning, you can basically just swap drives and be done.