There are a couple of issues with a wing that is too big. One is probably minor in most circumstances, and that is drag. But the other is not minor, and that is venting. When the wing is too large, it will wrap up ("taco") around the tank. The air in the wing is then at the very top of the arcs of the wing, and is very difficult to vent. It may require big changes in orientation underwater to get the air to any dump valve.
Even for cold water use, a 30 to 35 lb wing is adequate for most people using a single tank. The lift needs to be enough to float your rig at the surface (eg. if you have to take it off before getting back in the boat) and to compensate for the lift lost by your exposure protection at depth. Especially if you are small, even using a thick wetsuit in cold water, you're unlikely to lose more than 30 lbs of lift.
The choice of adjustable versus one-piece harness is a personal one (unless you're DIR

), but the one-piece harness is cheaper and, in fact, more adjustable than the complicated ones. A lot of people buy the fancier harnesses to begin with and find they don't work as well as they had hoped, and end up with the cheaper and more flexible option as a second purchase.
Good luck with your class and your gear -- Scuba is great fun!