Start with KB2919442, the March 2014 servicing stack update for Windows 8.1 and Windows Server 2012 R2.
Then you need to install KB2919355, the April 2014 update, which is a package made up of six separate updates plus an update tool. These must be installed in the following order:
You can then install the latest servicing stack update, as shown in ADV990001.
Finally, install the latest monthly rollup, as shown in KB4009470 - look at the update list in the left-hand margin; the latest rollup should be near the top of this list.
Unfortunately, the monthly rollup updates for Windows Server 2012 R2 are not comprehensive. If you want to bring the new server completely up to date before putting it on the network, there are a number of older security updates that should also be applied.
On my system, these security updates were detected as needed: KB3185319, KB2973201, KB2976897, KB3010788, KB3011780, KB3019978, KB3021674, KB3023266, KB3035126, KB3042058, KB3045685, KB3045755, KB3045999, KB3046017, KB3055642, KB3059317, KB3061512, KB3071756, KB3082089, KB3084135, KB3086255, KB3109103, KB3110329, KB3126434, KB3126587, KB3133043, KB3139398, KB3139914, KB3146723, KB3155784, KB3156059, KB3159398, KB3161949, KB3172729, KB3175024, KB3178539.
Non-security updates: KB2967917, KB3000850, KB3008242, KB3034348, KB3042085, KB3044374, KB3077715, KB4486459, KB4490128, KB4501226, KB4339284, KB4462901, KB4468323, KB4033428. (Plus another 66 optional updates.)
Also new versions of Microsoft .NET Framework (and associated updates) and the Windows Malicious Software Removal Tool.