Mistrals and Stream Airs are basically the same regs, with the Mistral being a slight upgrade to the SA. They are the most basic kind of diving regulator. Both are single stage regs, meaning that the air they supply to you goes directly from tank pressure to ambient unlike 2 stage designs which regulate tank pressure to some lower pressure then supply that to the second stage which again reduces it to ambient. There is no place to use a second stage (octo) or even modify the reg to use one. Like wise, there is no place to add a SPG, this is diving at it's very basic design. Both regs were designed to be dove with J valves. You can "add" a spg by placing a device called a banjo between the tank and regulator if your reg has a "long yoke". The long yoke was used on the later models and was an option for adding to older models, most do not have this feature so you would have to replace the yoke to use the banjo option. Bryan sells long yokes and banjos if you want to go that route. The only other upgrade available to them is the duckbill eliminator. That said, all the parts you need to rebuild them to better than new condition are available and diving them is a lot of fun. Most of us have at least one we dive.
Here is Eban showing the banjo and the differences between the long and short yoke.
https://www.youtube.com/watch?v=Ey5els2wpKs
If you want to ease into DH diving and have modern features like a SPG or octo then you will need to get a 2 stage design, the DA Aquamaster or Royal Aquamaster. Those have a lot of modifications than can be done to them to add SPGs and LP devices.