I've played some 4th Edition, but I'm curious about trying an all-Essentials game. My understanding is that I need the appropriate Heroes books (in addition to PHB/DMG/MM). Are there any meaningful differences between the Guide and the Kit?
The Dungeon Master's Guide is a book published at the very beginning of the 4e line. It is a comprehensive rules overview.
The Dungeon Master's kit is a box set part of the Essentials line that contains a Dungeon Master's book, an adventure (Reavers of Harkenwald (which is a great adventure btw)) and a set of tokens.
The Dungeon Master's book from the kit has a lot of the same content contained in both DMG 1 and 2 including all of the rules updates they had done up to the publication date. However, while I haven't read it through it is shorter than the two DM books combined so it would seemingly have less content (I've flipped through it a couple of times, and there is a lot of good stuff there, but I can't evaluate it fully).
If you are running an all Essentials game and want to run through that line of products I'd say that getting the Dungeon Master's kit would be a great place to start. It sets you up with everything you need to run Reavers except for the character books (which are contained in the "Heros of" books quite nicely). You might also look into the Monster Vault as a continuation from there as it will improve your supply of tokens and has a mission that is sequential to Reavers (or that can be fit into an extended campaign).