Are you planning to do the work directly on the laptop's screen, or with an external monitor? (In other words, do serious CAD work at a desk, since you're going to need a mouse anyway; use the fact that it's portable as a reason to move between desks, and only rarely to edit something on the laptop's own screen.)
If you can't acquire a cheap 24 in LCD monitor for CAD work, and thus need to work directly on the laptop's screen, the T510 would be a reasonably good choice if you're planning to use the laptop every day. (With the professional-grade Lenovos, HPs and Dells, you're paying for durability and serviceability.)
If you're just going to be doing CAD occasionally, and doing it without an external monitor, look for a consumer-level media-capable laptop with a Core i5 or i7 mobile processor, a higher-end GPU (though not necessarily a professional-grade one like a FireGL or Quadro) and a big screen (with high resolution). You might want to try
this for ideas.
(I personally like my X201 for most things, although I'm doing a lot less CAD nowadays. It's very fast and very portable. But then again, I use it with a docking station and an external monitor whenever possible. It's a bit out of your price range, anyway.)