Lenovo and Dell are both good. They are generally the ones I use. If you use SSDs, at least 8 GBs of RAM and the latest i7, you would be pretty well set.
You can consider 16 GBs of RAM. The main reason is if you install or use a VM, you will have extra RAM to allot to it. Sometimes, it is good to test programs such as AD in a VM. Sure, you can test it in a sandbox, but then you have to remove AC from that PC and install the newest version with is own SQL.
Much easier to install it to a VM where you can play with it at your own desktop and use snapshots to go back instantly if you screw it up. I would highly recommend that.
