I'd recommend brewing a mid 40s gravity batch first and using the entire slurry from that.
So in the absence of a batch of slurry what are your thoughts? My plan was to make a 2 quart starter today, then add another quart of starter wort to the original to give me a gallon starter.
How much yeast should I add at bottling?
What he is saying is that you should brew a whole mid-40s (1.043-1.047) beer. You could do something along the lines of a Kölsch, a dry stout, or even an American Wheat beer.... Then pitch your Imperial Stout on top of that slurry. I would even go as far as brewing something in the low 50s, like a 1.052 pale ale, before pitching the full slurry.
As for the yeast to add at bottling, just pitch a pack (or maybe 2 in this case) of US-05 a day or two before you bottle. Some people recommend bottling with Champagne yeast in a beer that big, but I have never done that.