Can you buy Wii U downloadable titles without a Wii U?
Solution 1:
Currently you can't, Nintendo ties the download with the console, not with the User ID.
Solution 2:
Update since the answer has changed. Unlike three years ago, you can now buy titles on Nintendo's website, and they will email you a download code. You can save this code and enter it in on a Wii U console at any time later to download the game. The same system applies to 3DS.
Nintendo Support link on giving games as gifts (though you can also just use the code yourself).