I spoke with a contractor and they seem to indicate I could use the existing stack (3" that is the drain for my kitchen and 2nd floor bathrooms (2)) to vent the toilet and shower but I would need Studors on the other fixtures
You cannot vent anything into a waste stack. Now if that waste stack extends on up through the roof you can tie into it, but not until it is at least 6" above the flood level rim of the highest fixture served. In other words, if the stack you want to tie your vent into serves the second floor then you cant tie into it until you're above those fixtures.
As far as the tie in goes, it really shouldnt be an issue what angle the building drain is running at in relation to the proposed bathroom as long as the proper fittings are used and installed correctly. After all, thats what they make fittings for, eh?
thekctermite brings up a very good point about the backwater valve. If your building drain is below the next upsteam manhole then you need one. Thats something you really need to look into now. It blows when you have everything in then get turned down on your inspection because of something as simple as that.
Good luck and keep us posted