The set that will come from this is going to be a limited time gift with purchase when you spend a certain amount of money on Lego.com or in Lego stores. The purchase threshold for Lego Ideas gifts that you will have to meet usually tend to be around $120-$150.

This isn't going to be a full fledged theme and the minifigures and set that will come out of this will be for the most part determined by the design of the Lego Ideas contest winner.

In addition, gifts with purchase tend to be the same size as $30 sets or so, meaning it's not going to be a very large set. So just keep expectations in check.

TLDR: it will be a small set you can't purchase outright.