I like to smoke at 215, 250's just too high for brisket IMO.
Mustard's usually saved for pork, Olive Oil, Canola Oil, or thick Worcestershire Sauce are most commonly used on beef. There are those who claim that oil on the surface of the meat will prevent smoke penetration. I am skeptical of this claim, because I rub all my meats down with olive oil, and have never had an issue. Furthermore, if you do a spice rub, oil means smoother transfer of oil-soluable flavors from the rub to the meat. I've had brisket and chuck that got the mustard rub, and it didn't really lend a different flavor or texture to the meat, so I considered it a waste.
Rub: keep it simple the first time through, experiment later. I would avoid rib or pork rubs as they're too sweet for beef. SPOG (salt, pepper, onion, garlic) are the basic coverings of brisket. I also like cayenne.
Rubs take time, so make sure you coat your brisket, and let it set in the fridge for at least a few hours. Overnight will be best.
You can spray it as you smoke, but Webber says that every time you open you smoker to spray/fiddle with the meat, you add 20 minutes onto the cook time.
I would advise searching the site for Brisket posts to see how everybody else does it so you can formulate a plan, but BearCarver and SmokinAl have some good threads about brisket. BBally's got a good few, too, if I remember right.
Also, Meathead's got a great teaching site:http://www.amazingribs.com/recipes/beef/texas_brisket.html
I read his methods up one side, and down the other for a month or two before my first brisket.
Many folks foil up their brisket at 160-ish, and let that braise away until 190-205 degrees, temperatures will depend on preference. Closer to 195 means a little tougher meat, but it stays solid better for slicing, closer to 200/205 means a more tender product, but runs the risk of drying out, or shredding when you slice.
Finally, when you pull the meat from the heat, foil it, wrap it in plastic, wrap it up in towels, and park it in the cooler for at least one to two hours to let it finish off. You'll be glad you did!