Picking out a good brisket...find a good limber one, one that you can bend easily while still in the cryovac. Look for good internal marbling within the strands of meat. The higher the grade of meat, the more internal marbling will be present, however, the briskets aren't graded individually, the grade comes from the carcass it came off of. So at times you might find a jewel in a Select grade that may be as good as a Choice or even a Prime. Buy as high grade a meat as possible, but don't fret if it's just a Select, it will turn out just fine, just cook it right.
As far as cooking whole or separated, either way works just fine. The flat is where slices come from typically, while the point can be sliced, cubed or pulled and chopped. I have done briskets any which way you can think of (whole, separated, no trim, heavy trim), it all comes down to cooking them right. Smoke for 4 hours, wrap in pan with some broth, beer, soda, your choice. Cook it until it reaches above 190 IT, then every 5 degrees after that, check the brisket for tenderness by using a toothpick. Probe the flat portion of the brisket in several places, once the toothpick slides in easily with no resistance (like a knife in butter), she's done. Once done, wrap in towels and throw in a cooler or at the very least on your kitchen counter top. Once the IT has dropped to below 170, slice and enjoy.
Best of luck and enjoy the smoke!