There is not really a specific time to buy since retailers have sales year round. You just have to keep looking. Also, you don't need anything crazy for engineering because you won't be running CAD programs and such on it. The college should have desktops for that.
Laptops with Intel's Haswell CPUs are coming out in Q1 2013 (or early Q2 at the lastest). Therefore you can expect prices of laptops based on Ivy Bridge to drop a bit to clear out inventory for these new laptops. I would expect prices of current laptops will begin to drop a little bit in January 2013 and may drop a little more (may be) as the release date of Haswell based laptops approaches.
Other than that you would probably need to wait for August for the back to school sale to buy a Haswell laptop at a cheaper price.
My current laptop, an Asus N61jq seems to be sufficient to me for now. I'm preferring a 14" or 15" laptop, though I'm not sure what laptop weight would be the best for a college student because I know I have to haul a lot of books based on my older sister's experience. However, I still do want some gaming capability without murdering battery life.
Playing games on battery will drain the laptop quickly even though by default when running off of the battery the CPU and GPU will run at slower speeds. You will typically get around 45 minutes to 75 minutes of game time before the battery is drained. The more powerful the CPU and GPU, the faster the battery will drain; which means.... shorter battery life.
I've heard that many gaming laptops still drain their batteries quickly even if they were set to the lowest power setting. I don't expect great battery life when gaming (most likely I would be plugged in), but I do want the laptop to last when I'm doing lighter tasks such as typing down notes.