I don't believe it's possible to partially download a playlist. However you could create a playlist with only the songs you want available offline and then download this one instead of the original with 800+ songs.
I have the same problem as well. I would also suggest the option to click a button to automatically download say the next 10 songs from the current queue, so that you can just click that when you go out of the door and keep listening until you get to your next WIFI spot!!