Don't buy prebuilt rigs. Most have 1 really good component, 2 so-so ones, and the rest are usually trash because people who purchase these things aren't computer savvy so the manufacturer/seller stacks the computer with crappy hardware. You also have to be cognizant of component warranty if you're going to do a lot of rendering, and prebuilt computers are notorious for having short warranty times for some if not most of their parts.
Try to build your own rig, even if you have to order the parts and wait a few weeks. It'll save you a ton of headaches down the line.