where do i download el capitan os x 10.11.6

If you do have an operating system on your mac and it is functioning then you can download

El Capitan using this method.

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the .pkg to the Install OS X El Capitan.app which you will find in 

your Applications folder, it should be 6.2GBs in size. 

If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time.

Not any internal or external disk that you want to eventually install El Capitan on, 

that is for later.

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk, using the Install OS X El Capitan.app

in the Applications folder, and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

1. Yes, files should be fine, apps will need to be compatible with the OS you are running on the SSD, if they are you can move them over or install them on the SSD, for better response, and makes more sense.
2. USB 2.0 is alright, about the same speed as Firewire 400, any connection is only as fast as its slowest end.

3. Never call yourself 'only a drummer'.

Click on Desktop on Finder top left of your screen select Preferences.

Click on General and select to show Hard disks on the Desktop.

Then click on Sidebar and check the boxes to show which Disks you would like to see in the Sidebar of Finder windows.

With those two items set you can then navigate through the internal disk either by clicking on the internal disks icon on

the Desktop, or by clicking on the internal disks icon in the Sidebar of a Finder window.

No I am not a drummer I am Ringo Star. (Boom,Boom)