If you want to keep your costs low (which I recommend any n00b to any setting, or game), then the 4e book is already more than enough.
It's specifically designed to let you run the entirety of the (3e) MMRPG adventures - which you can find, in their entirety, and for free, here:
viewtopic.php?f=38&t=2827With that, you should already be good to start a game that feels "authentic", and you should be able to run it for quite some time. :)
My personal recommendation, if you have no experience at all with the setting so far, is to get the unjustly-reviled "Dungeons of Castle Blackmoor" book that was released for 3e, though, and to mimic the original Arneson campaign, for a while, at least: Start with the town and the castle, and add things according to the needs of your game.
Or, if you want to stick with the MMRPG modules (all assuming that you don't have a campaign idea that you want to follow yet), I recommend going either the Heronport adventures, or, in all modesty, with the adventure I myself wrote for the MMRPG - "Tales from the Old Land":
viewtopic.php?f=38&t=2827I think "Heronport" and "Tales" are the two most beginner-friendly adventures, among those that you can get for free, at least. The Redwood series is also pretty good, but not free, of course.
- Obviously, what Rob and Aldy wrote is also true: Really, all introductory material, for all editions of BM is so good that you can't really make any mistakes, no matter your choice. I also recommend you to spend those 20 bucks for the DA series in PDF; however, if you are a 4e DM, and already have the 4e book, it's sort of a lot of stuff to read.
My recommendation: First, get an overview. Then, work with the free - and, in part, extremely well done material - from the Heronport adventures to (ANDYEEEAAAHIAMPLUGGINGHERE) The Age of Theodore, AND THEN spend more money, expanding things specifically according to your needs. Most free stuff, when it comes to Blackmoor, is at least on par with the officially published material - and that is not at all meant to question the quality of the official publications. It's just that the fans did some really good work. :)