If you haven't the cash to buy a mobo,ram and CPU bundle in one go, wait until you CAN afford to. If you need to scratch the itch then a case is the best option as you can inspect it right away. Buying the CPU first means you can't test it and it might be faulty. Then you're out of returns to retailer timeline, so you'd be dealing with manufacturer in that instance. A case you can see and feel it when you get it, thus able to deal with any issues from the get go.
While I agree with the point, the logic in this case doesn't apply, intel RMA's have always been obscenely quick, I had a new NUC 2 days after I sent it back (shipping would take a day each way), CPU returns (though very rare) are equally fast.