Q: Reinstall OS originally come with Mac Pro 17 inc Early 2009

Hi, I guess that would be helpful if you guys out there can help me address the solutions.

The problem is that I bough used macbook pro 17 inch early 2009. The OS installed is El Capitan. I found that it does not function well and I want to reinstall the original OS that come with this mac (10.5.4). However, I don't know that background of installation to which OS version has been installed since this mac has been purchased from one to the other's hands and upgraded.

It does not come with the OS DVD. I try to do internet recovery but seemingly it does not support 2009 models Computers that can be upgraded to use OS X Internet Recovery - Apple Support. Nevertheless, I pressed option+command+R and then new window appeared. I followed the instructions and so on. Then it say OS X El Capitan (software license agreement).  I clicked agree and I choose HDD to install as it said "reinstall a new OS version". After I hit continue, It took forever to download additional files whatsoever.

Is it tricky and difficult to reinstall mac 17 early 2009 but run EL Capitan?

Then I am trapping as I cannot even go back to run the original OS that come with this mac nor I don't have OS DVD. If so, is there any solution that avoiding buying OS leopard (as an original OS)?

Your assistance would be much appreciated.

Out