Once upon a time, they said that to do NaNo right, you had to start a new novel. And I like to recommend that for newbies, even now that they've changed the rules, because I feel like the big build to NaNo is much better when you're revving up for something new (or something you haven't worked on in a long time), rather than continuing something you've been working on for the months leading up to NaNo.
However, in reality, I recommend working on whatever is exciting you. Whatever state it is on day 1 or how salable it'd be in the long run or how many drafts you'll have to do in the future. As long as you're motivated, go for it and have fun!