you can choose to marinade or not, depending on what you are looking for in the final product. some folks keep it simple and marinade with one liquid, such as beef broth or dr. pepper. others marinade with an oil + and acid such as wine. for a first attempt, i'd recommend keeping it simple so that you can start at a fundamental level and build on that experience.
for cooking the brisket, i would recommend temepratures between 240-250 degrees on the grate. you can go down to around 230 if you want, but i wouldn't go much lower than that, personally, as with such a long smoke there is a danger of drying the meat out - plus, there is something that happens to meat at temperatureas around 240 or so and above. i think it is called the maillard reaction, but in laymans terms, it simply gets good.
a mopping baste during the smoke is recommended, starting about 2 hours into the smoke and then again every time you are fiddling with the fire, opening to check temperatures or whatever. once again, a mop can be pretty much anything you want, but should have some sort of oil-base. last year i used a mop that was a 50/50 mix of beer and melted butter (blend before each application of mop, in order to emulsify the ingredients) with excellent results. another one that i have had great success with is a mop consisting of 1 cup dr. pepper, 2/3 cup low-sodium soy sauce and 1/3 cup olive oil. this is something that really brings out some great flavors in meat, plus, it can be modified to push flavors in a particular direction by changing any of the ingredients around a bit.
as far as wehen your brisket is done, there are many schools of thought. i have not yet perfected that part of brisket cooking, so i will eave it to the experts to answer that question. the temperature i hear the most batted around is 185-190, then pulled from the heat, wrapped in foil, and rested at least a half-hour before slicing.
RIVET has an outstanding finishing sauce for brisket and i highly recommend it. here it is: