Firstly welcome
My first scope was also an 8" dob and I can tell you that straight away with the plossl eyepieces that come with it you will already see great views of saturn and its rings. This will be improved over time by adding better eyepieces to your collection.
You will see some nebula, but the thing to remember is you won't see them in colour like in all the pretty pictures.
But an 8" dob is a great place to start that will give you loads and loads of fun and plenty of things to see. I assume from your budget you're not getting a goto scope so you'll be manually controlling it, so get yourself some star charts to help you learn the sky and there are many free programs for your pc or apps for smartphones that can assist you in this area also. For example stellarium for the pc (and iphone i think) and apps such as sky safari or star walk to name a few off the top of my head for smartphones.