Rega P2 or ...?

not that my opinion means anything to you.....but when i had the Belles Aria, I didnt think the output for the phono was enough, so you might want to look at a external phono preamp.

Do you think a Rega P2 hooked into the pre on my Belles Aria Integrated will sound better than my bluesound node 2i (amazon hd) into my schiit Gungir multibit?  If no, then at what level will the turntable sound better than my digital setup?
The Rega P2 can be had for $400 so I am considering that turntable...  

no.     you are comparing analog to digital ..          dont buy a turntable just because its in your price range.....you wont be happy and will want to upgrade soon after.                 if you want to get a nice turntable setup, then plan on spending  $1500-2k , which includes turntable, cartridge, phono preamp, phono cable .

do you know how to setup a cartridge ?

$400 for Rega P2?! If it's in good condition, definately worth auditioning at that price, as you can easily get that $400 back by reselling it.  

I can't speak for the phono input on the Belles, but I have a Bluseound and purchased the P6 with the MC ($2300) and that matches and usually beats the sound in my opinion. I do have a really good phono preamp in my integrated amp also however. I agree with Riley basically, I would not spend less then $1.5-2K on a table and cartridge. Then add preamp and cable if desired. Otherwise without a solid pre-amp, I would say it would not match the sound of the Bluesound.