I'm having trouble finding a good MacBook pro battery that has an original voltage of 16.5V 3.65A max. Even for any laptop, I can't find the one with a voltage of 16.5V. Unless the information on the back of my MacBook is not the correct info I should be looking at. Also at this point should I even fix and upgrade this Macbook pro in late 2011 13 inches.
The indicated voltage is not a problem if you can find a compatible battery.
If you search on the net for "A1278 Battery Macbook Pro" you will be spoiled for choice. On the other hand, it's a bit of a lottery and you should not expect to find a quality equivalent to that of the original batteries.
For the second question, on a 2011 MBP model you can put 16 GB of Ram and an SSD. You should still know that the 13" and 15" Late 2011 models (unlike the 13" Early 2011) suffer from a frequent problem with the graphics chip on the motherboard.