What cassette would you buy to make all this work together(if it's at all possible)?
Shimano crankset 105 hollowtech II.
105 front mech
Sora rear mech
Campy mirage ergo 8x2
You can get that to work if you buy a shiftmate.
Wow, what a collection of bits!
I'm sure there's a way it can be 'forced' to work, but it won't be pretty, there are devices that change the shifting from campag to shimano, what sort of cassette do you want to run, how many speeds?
Personally, I'd get off the fence and go all campy or all shimano.
you need this: http://www.ctc.org.uk/desktopdefault.aspx?tabid=3946
Try the CTC forum. They are much more geared up to this sort of thing over there.
For what its worth though, it will work fine with an 8 speed cassette. Spacings on the cassette are 4.7mm for Shimano and 5.0 mm for Campag.
I have used that combo for many bikes. My current "roughstuff" bike is running a elderly 105 rear mech with even older Record 8 speed shifter. Fine. The fact that the left shifter will shift in small steps will help the front trim if its needed. I bung a variety of chainsets on and all I do is adjust the stops on the front mech to suit.
As you are all but committed to your set up anyway just buy a cassette of your chice. Get a cheapy. Be careful about the capacity of the rear mech but most Sora stuff I see seems happy with anything other than a whopping big 34 rear.
I am guessing you are going double at the front so you won't have a big difference there.
If it doesn't work perfectly look for shiftmates.
SJSC are good for that sort of thing.
Many people say that it will get sloppy when it ages. I think it improves as a touch of play in the chain more than makes up for the 0.2 mm differences in cassette spacing.
To be honest 8 speed mixes work fine.