Reviving 4,1 Mac Pro

I posted in r/Apple the other day - have since found this sub. I've decided to make a lifestyle change to using Macs - mainly because I've started using Macs again at work, which has reminded me of how awesome OS X is - and Windows 10 is a catastrophe buried under a seemingly finished OS. I hate it.

I picked up a 2009 Mac Pro 4,1 with 12Gb RAM, but no hard drive for a low price of zero dollars. New drive is on order. I had a little chat with Apple today to see how I could get OS X installed on the machine. The rep said using Internet Recovery should do it. So Mac gurus...will it work? From what I understand, I just install the drive, plug in a network cable, and turn it on while holding Command+R, and it will go to work. Then run the disk utility to format the drive - and it will then install OS X.

I know the machine is well qualified to run El Capitan. Is it a free update? I'm not familiar with how Apple does its' OS upgrades.