The good news for anyone looking to purchase Disney+ is that it won't have any ads ever, according to Disney themselves. However, Disney will be offering a bundle package which will include a subscription to Disney+, ESPN and Hulu, but the Hulu service will come with ads.
However, Disney may one day decide to turn on ads for their Disney+ streaming service. There is speculation that the reason Disney are offering the version of Hulu with ads along with Disney+ and ESPN is because they want people to get used to the idea of seeing ads alongside whatever they're streaming. This is actually not a bad idea, as it could allow Disney to launch a version of Disney+ with ads for even less money than the current price.
So although Disney+ is currently available without ads, we may see them on there in the future. Keep an eye out!