I wanted to buy this iMac. Does anyone know if I could upgrade the RAM to 16?
This particular model that you're looking at is referring to the Late 2013 model of the iMac, 27" display version (model number ME089LLAUZ from the linked store page and verifiable through Apple's own KB article).
Apple sells this on their store still, and you can order a custom configuration with up to 32 GB of RAM as specified on its product page. This would probably be the better option after a glance at the numbers when you add 16 GB of RAM to the order.
However, for the model on the website you referred to, you can also purchase supported RAM yourself and install it in the two empty slots at the bottom of the machine (Apple has their own guide for that here). Note that the RAM you purchase must meet the following criteria (may seem like a lot, but compatible parts are easily found on Amazon and other suppliers):
- 1600 MHz
- DDR3 SDRAM