For the crockpot, put it on low. Sliced or whole doesn't make to much of a differance other than the sliced will heat up faster, but the end results are about the same. The main important thing is to save all them drippings from when you cook the brisket and use those plus additional sauce, liquid, broth, ect. ect. (if you need additional that is). You want approx 1-2 cups of liquid in the crockpot, depending on how much meat you have.
A great way to get some awesome ajus when cooking that flat is to put it in a covered foil pan with bottle of Killians Irish Red Ale when the internal temp hits 160°. The Killians adds a nice subtle flavor, that goes really, really well with brisket.