So STEEM and Steemdollar are two different currencies, but they operate in the same block-chain (social media based block-chain).
They have different use on the blockchain. So Steemdollar is mainly to hold, but you can invest in promotion of your own posts with steemdollar so you you can show up on the promotion page. Different amounts will determine how high on the promotion page your post will appear.
You can also use your Steemdollar to invest in STEEM or STEEM Power as well. When posting, you can also choose to only want to earn only STEEM Power, a combination of STEEM Power and Steemdollar or just Steemdollar.
Dtube operates just like steemit, where you can post content, comment and upvote. All content uploaded on your Dtube will be created as a post on Steemit, since it is the same account you are using.
If you read my post about investing in STEEM, then you will understand that investing in STEEM Power would be a great move to increase your influence on the platform.
the right time of buying Steemdollar depends on what you will use it for. if holding, then I do not really know... if using it on the platform, for example to promote, then maybe buying steemdollar could be a good move.
I would say that it is better if you managed to earn all your STEEM and Steemdollars without buying. But of course, if you are looking to do big on STEEM platforms, then buying and investing in STEEM could be a smart move.
I hope this helped you out in some way, and if not, then just ask and I will gladly answer your questions.
I post some content around this topic if you are interested in reading. maybe consider to follow as well :D
I wish you a well STEEM journey!
RE: A few questions about the Steemit platform !