I would agree with others about needing a numeric keypad. It helps in many situations. As for a laptop I got a used hp envy m6-1205dx http://www.amazon.com/HP-m6-1205dx-discrete-class-LED-backlit-high-definition/dp/B00BGYFGKI, got mine from ebay for about 450-500.
I also bought 16gb of ram and it works great for unity. was also able to do that and get a 250gb ssd last year for under $700 US. I would bet you could put together something like that pretty cheap especially now. only things I didn't like about it was only 1 hd bay, had to get an external enclosure for the 750gb drive that came with it, and i would have preferred a larger screen.
I would check out the amd apu based laptops if you are looking for a bargain laptop for game dev.