I don't see the need to buy more storage now unless you want to download full retail games. Then you're going to need to get an HDD. Are full retail games available via the eshop yet? I don't know as our Wii U is for Xmas. Though I'll probably fire it up and update a week before Xmas or so.
Yes quite a few retail games are available digitally. Honesty right now is the best time to buy an external HDD since you have the Black Firday/Cyber Monday deals going on and a few places have external HDDs on sale for that or this week.
I am enjoying having the 1 terabyte HDD that I got for $20, just in case I ever do need that much memory but for $20 why wait to get it later. Of course I do only have a 8gb system though.