LENOVO W530 INTEL HD GRAPHICS 4000 DRIVER SERIES
W520's and T520's start from $500 or $600, while used L series ThinkPads start from probably $400 (maybe even $300 - not completely sure, I rarely see them in the forum's marketplace). You'll be able to get a W500 starting from $300, a T500 starting from $200, an R500 starting from around $100, a W510 starting from around $400 or $500, and a T510 starting from $400. (I'm actually planning on getting a T500 - or even a W500, if I can afford one - soon, which I will be using for DJing.) Next would be the T500 and newer models (T510, T530, etc.), then the R500/400, and lastly the L series. The W510 can support up to 16GB of RAM, while the W520 and newer models can support up to 32GB, and the W500 (which is the first W series model) can only support up to 8GB. It is the top-of-the-line ThinkPad (its a mobile workstation) with powerful GPUs and CPUs (up to Intel i7 extreme processors). The W series would be #1 on my list (personally, if I had the money I would get one of these). You can also try the Lenovo Outlet - where you can get one of the newer ThinkPads new, refurbished, or slightly used (for a lowered price).
(If you don't already know) You can always get a used ThinkPad here on the forum for a good price. I think a T, W, or L series, or R500/400 would serve you well. If you want to get a ThinkPad with on board USB 3.0, you will have to get one of the newer models (T530, L530, W530, T440/s, etc.), since that when ThinkPads started to have USB 3.0 ports. Nice to see someone else who's DJing with a ThinkPad!