I would consider the ThinkPad E550 which has a base price of $579. It comes with a Haswell generation Core i3-4005u @ 1.7GHz, but the new Broadwell generation Core i3-5005u @ 2.0GHz is a free upgrade which is not only more powerful but also consumes a little less power. However, I would upgrade to the Broadwell generation Core i5-5200u for $100 which has a base clock speed of 2.2GHz and a max turbo boost speed of 2.7GHz.
http
/shop.lenovo.com/us/en/laptops/thinkpad/e-series/e550/
It only comes with 4GB of RAM, but you can upgrade to a total of 8GB for $80. Or you can simply buy a stick of 4GB PC-12800 DDR3L 1600 1.35v RAM for around $40 or so and install it yourself. The screen is only 1366x768 which is too low for productivity purposes in my opinion, therefore I recommend upgrading to a 1920x1080 for $80.
The 500GB hard drive can be upgraded to an SSD (which is much faster than a hard drive) to load programs faster, but the only option is a 128GB SSD for $130. I recommend you simply get the laptop with the 500GB HDD and buy a SSD on your own and replace it yourself if you feel that the hard drive is too slow for your taste.