newbie seeks help with system design

Honestly, being as it is B&O then I would call Peter Ledermann at The Soundsmith as he is probably the man in the country to see about that particular cartridge. Because in your case you have what you have and don't want to have to buy anything you don't need. The cartridge in that case determines somewhat the phono stage. Ledermann also does repair and refurbishment and may even have a phono stage in stock. You never know. At the very least you will be talking with one of the giants in the field. 
https://www.sound-smith.com/lookup/bo-cartridges/133

Other than that all I can say is set your budget, make sure you have enough cartridge output for the phono stage, then pay attention to listening impressions more than anything else, and don't forget to include a decent quality interconnect in your budget.

The Project Phono Box is a good phono stage for your system. Don't replace the Acurus/Bryston pair! Sonos is good for streaming only! You might consider adding a better cheap DAC later (Topping D30 - $300). Cables and speaker wire can be entry-level Audioquest or Kimber.

Would not hurt to follow the advice provided by millercarbon. Cartridge output + phonostage gain = desired output. 

Might want to check out threads on the Bluesound Node for streaming.

HI,
Consider checking your tt first as it is in its third decade. Check what cartridge model it carries and investigate for its replacement stylus, some of their mm designs are pretty good and a perfect match for your arm.

Being a newbie probably should explain, according to what I found the cartridge that came on that table had an output of 2.2mV which should be fine with the MM Pro-Ject Phono Box DC phono stage you mentioned. Basically any MM stage should be fine. 

As for streaming, its doubtful you'll find anything in your budget to match what you have. Then again what you have does not seem to include any interconnects or speaker cables. So at the very least you are looking at an interconnect between the pre and power amps. 

In the price range you are at it makes no sense to be buying separates that need more stuff like that to sound good. So much depends on budget, and future plans. Everything has its pros and cons.

The ProJect phono stage for example is probably very good for the money and being a separate component can be easily upgraded. But how good it sounds will depend on the quality of interconnect used and that's another budget item. Theoretically you could find an integrated amp with built-in phono stage that will sound a lot better than what you have now. But then if you want to stream now you're talking some kind of digital in there too, or a separate outboard device for streaming. 

Its choices like these made me pretty much abandon digital. Because it makes no difference really whether we're talking $100 or $10k or $100k the choice is always one or the other. What it really comes down to more than anything else is how will your system be used, which comes down to what kind of listener you are. Get a handle on that, learn a few basics, and everything pretty much falls into place.

Sonos is overpriced for dummies.
Stay away.

Sonus is not that good. If you want a nice entry streamer, look for a used Auralic Aries Mini.
I have had that B&O turntable decades ago and the cartridge is the weakest part. IMO, if you have to rebuild the cartridge, then I wouldn’t do it because the TT isn’t worth it. Also, the TT setup with a cartridge that is playable, it doesn’t sound that good. If you want to get into vinyl, you are going to need to spend some cash before vinyl sounds better than the equivalent amount of cash spent on a separate dac. I have had many TT setups ranging from $100 to a recent $15k setup, and my digital setup for 1/2 the cost sounded better, so I sold all my analog stuff. If you don’t have a vinyl collection, invest your money in a decent digital setup. If you have a large vinyl collection, then save your money and get a decent TT setup (table, arm, cartridge, phono preamp) that will sound good