You're actually asking a lot of the wrong questions.
To answer what you asked:
1) should marinate it? If so what works well? What about brining?
Don't bother marinating. Salt and pepper is the basic rub for brisket. You can get fancier if you want to, but it doesn't help much. If you want to play with wet stuff, inject beef stock.
2) is their an estimate to lbs/hour?
It really depends on the temperature you're cooking at, but in my experience these numbers are worthless for all but the most rough guess. My rule of thumb--I cook at 250F, and it will take at least 12 hours to cook a packer.
3) whats a good temp to cook to? I will be slicing it.
This is exactly the wrong question. Cook to 185, then take your probe out of the meat. Seriously. The only thing you use a temperature probe for past this point is poking the meat every 20-30 minutes to see if it's done. When it slides in like it's going into warm butter in several places around the brisket, it's done.
4) to foil or not to foil? I know on ribs using foil works wonders for me
Foil does two things--it gets you past the stall (because you begin braising instead of smoking) and it softens your bark. It can help you get the cook done more quickly, but it's not necessary. Your brisket WILL turn tender without it, it'll just take a bit longer.
Now then for the things you need to ask but didn't know:
1. Temperature control is everything. Whatever temp you cook at, you MUST hold close to that temperature. Brisket doesn't stand up well to a pit that starts at 250, runs up to 290, dips to 230, etc. When I'm cooking brisket, I target 250F and set my Maverick to alert at 235F on the low side and 265F on the high side. I've found that staying within that 15 degree range of 250 is fine, but going further isn't.
2. It's done when it's done. I know you've heard that a bunch before, and the saying was invented for brisket as far as I can tell. Lots of other cuts of meat are done at a certain temperature, but not brisket. Brisket it done when it's tender all the way through. The only way you can tell that is by testing it. Some people use the jiggle method, others use the probe method, still others just give it a good feel. I like the probe method because it's easy. Poke your temperature probe into the brisket. If it goes in like you're poking warm butter (just the barest hint of resistance) it's done. If it doesn't, it's not. Unless it's over-done, which is why you start testing at 185, and test every 20-30 minutes.
3. Undercooked brisket is tough. Overcooked brisket is dry and crumbly. There's not much range in the middle. Also, if you're cooking a packer (and you should be cooking a packer, they're more forgiving), you may not have all parts of the brisket done at the same time. You have to get a feel for what is done and what may not be. Really what it comes down to for me is that the end of the flat may be overdone, but most of the flat and the point are perfect. I just throw out the overdone part, or save it for chili.
4. Let it rest. You can't just pull the brisket off the pit and slice it. It needs to be wrapped up and rested until the internal temp drops down to around 165 or so at least. The good news is, since you don't know when it will be done, you're going to start really early. If Christmas dinner is at 5pm, I'd cook at 250F and have the meat in the smoke around 10pm the night before. When the brisket is done (somewhere around lunchtime), I'd pull it off the pit, foil it, let it sit on the counter for about 30 minutes to cool off a little, then toss it in a cooler with some old towels on the top and bottom. It'll stay warm for hours that way, and it'll get a nice huge rest to make it juicy all the way through.
Best of luck! Just don't be disappointed if it doesn't turn out great. Briskets are hard because you have to control your fire and develop a feel for when it's done. If it doesn't turn out great, you can chop the brisket and mix in some sauce, or just make chili.
