I would go and buy at least a 32GB SD Card, because you should backup your nand if you are going to install/run any CFW, and the Nand backup can be pretty big. And not to mention the Games can be pretty big too. And if you want to install more then one game at a time, an 8GB Card will make things much slower. With a bigger card you can just put a few Installable games in the SD Card, then install all of them at one time on your WiiU without having to transfer and install the games one by one from your SD Card.
And if you want to use DLC, you will also have to use CFW. And the DLC for some games can be pretty big too.
I bought a 64GB SD Card just for my WiiU, I bought mine on Amazon for about $20 I think.
And you have an External HDD, so you don't need to worry about filling up your internal WiiU storage. So a bigger SD Card would be perfect for you.
First, you need to plug your external HDD into your WiiU and let the console format the HDD so you can use it to install and play games on the WiiU.
But you still need your SD Card to install games and to run any homebrew. The games go on your SD Card in installable format, then you run WUP Installer from your SD Card to install those games, before it installs the game it asks you where you want to install the games, either NAND or USB. If you have a USB HDD already plugged in and formated for the WiiU, then you can choose the USB option. It will install the games you put on your SD Card to your USB HDD. Then you can play those games from the WiiU Main Menu.
However, to install and/or play DLC or any EShop games, you need to be running CFW. Even if you are installing a game that requires CFW to play it, you still need to be running CFW to install it. Same goes for any DLC, you MUST be running CFW to install or/and play the DLC.
If you want to install homebrew on your WiiU, I suggest you follow Kafluke's Noob guide, a link to his guide is in my Signature. And please make sure you read his entire guide before you even touch your WiiU, and please make sure you understand everything before you try to do anything on the console. If you are having trouble understanding something, please post your question on the forum, we will be happy to help you with what ever you need.
I rather you ask a ton of questions your not sure about, then brick your console because you didn't understand something. So ask as many questions as you need to, until you fully understand what your about to do to your console. But read the guide first. The guide should answer at least most of your questions.