Your filling is basically a beef stew/casserole, so any cuts that suite long slow cooking are fine. They get cooked again while the pie is baked, so premium cuts will just be wasted
The lower grade and 'off cut' parts are what are most commonly used. Like rump, brisket, chuck, shank, neck etc
Using a bench top slow cooker makes cooking this all very easy. The beef needs to be fully cooked before making the pie. An eight hour slow cook is fine
You want the pieces to be a variety of sizes to fill out the pie shape, but make sure you have some at least the height of the pie filling. The larger pieces tend to be better, but that could be a matter of taste and culture
You need to get rid of plenty of fat before and after cooking. A mouth full of fat in a pie is not nice. There is already plenty of fat in the gravy and pastry, so trim off all you can
A good meat pie should have a solid layer of meat connected with gravy, not swimming. It should also not be so full as you can't eat it with your hands (for a single serve pie) without loosing half the contents
Puff/flakey pastry works best and holds everything together
Avoid overly strong flavoured gravies, you don't want to hide the beef and pastry taste
Before you attach the lid, place a slice of tasty cheese, or an extra generous serving of cracked pepper on top of the filling
A respectable pie looks like this - thanks to Ponsonby Pies (not made in Ponsonby anymore)
It is possible to over brine meat. If you leave it in too long it will get too salty.
If you use a more dilute brine it won't get as salty but you will wash out more of the natural flavor into the water as well.
You could submerge your turkey in its packaging in ice water in a cooler for a day before brining. You could even thaw the turkey in this manner if you made sure to keep it in ice water so it didn't get to 40F.
Best Answer
It would seem to me that the gravy could facilitate heating the meat without drying it out.
Pour the cold gravy over the cold meat, seal all with a cover or aluminum foil. Bake at 300F (150C) until the internal temperature of the beef is at least 140F (60C) and the turkey is at least 165F (74C). That's hot enough to satisfy the New York State Food Safety Division of the Department of Health. That's good enough for me even if the ultra-conservative FDA recommends 165F for the beef too (I'm finding conflicting information for pre-cooked commercial products).
I'm assuming your meat is safe to slice and eat chilled, but you are taking it through the "the danger zone", heating it up. So take all the way through to the other side (140F), and be aware of time. (2 hours in the danger zone, cumulative maximum).