If you want to go all out you can buy a flat like you said and brine/cure it to make corned beef then smoke it or you can just go buy a corned beef in the grocery store and smoke it to make pastrami. I just picked up a corned beef from the grocery store and used that and it was awesome. When they go on sale for St. Patties day I usually stock the freezer so I have them all year. Good luck which ever way you decide to go with.
Yes you will buy the flat of the brisket to make your pastrami from a corned beef. Now you might want to soak it if it seems a little salty. You should maybe cut off a piece and fry it up for a taste to see. Then get ready for some of the best sandwich that you have ever put into your mouth. Hey next time you should really try making the whole thing from scratch the corned beef and then the pastrami out of the flat.