DM screens:
Wet-erase battle mat and markers:
A pound of dice:
Get a chessex as fulganger said.
They are amazing. http://www.amazon.com/Chessex-Role-Playing-Play-Mat/dp/B0015IQO2O/ref=sr_1_2?ie=UTF8&qid=1407451534&sr=8-2&keywords=chessex
that is the one i got, it is the 2 by 2 or length of an armish.
Now the key is to not buy the expensive wet erase markers they recommend. They're overpriced. Buy crayola washable markers. They have big giant WASHABLE on the front. If you buy during back to school time, they are as little as 1$-1.5$ for 8 markers! Such a steal compared to wet erase.
Also baby wipes are amazing for them. Towel+ spray bottle of water is cheaper though.
Tiles are more fun for some than dry-erase.
Also fun are lots and lots of dice.
The Chessex mats are really awesome.
Get a mat made by Chessex. They're available on Amazon. Then get wet-erase (not dry-erase!) markers. You can then draw walls, traps, etc.
I own this one: http://www.amazon.com/Chessex-Role-Playing-Play-Mat/dp/B0015IQO2O/ref=sr_1_1?s=toys-and-games&ie=UTF8&qid=1335629766&sr=1-1
It's very durable, and assuming you don't use dry-erase markers it will last a long time.
To start playing you only need one bestiary. Get the others as you get into the game and want more variety.
No sweat. There's always the classic standby "Battlemat". It's what most of my friends use.
Party size: 3-4 is the minimum, 6 in my opinion is ideal, though CR is balanced around 4. 8 or more becomes difficult.
Starter kit: For 5e, there's Lost Mine of Phandelver, aka the Starter Set. It comes with some pre-made characters(if a player is not confident enough to make their own), a dice set, and a starter rule book, in case you don't have a PHB.
Other things to have: multiple copies of the PHB are probably overkill. One is enough. A DMG is not required to play, and is mostly helpful when you're building your own campaign. In terms of dice, you don't technically need more that 1 die of each type, but for speed of play it's probably best to have a d20 for each player, 3-5 d4, 3-20 d6 depending on the character level, 2-5 d8, and 1 d12. A battlemat is useful for drawing maps for combat encounters, but you could also use graph paper, or even just verbal descriptions if you prefer. I'm not sure what a plotter is, but you could probably use that as well.
Game length: really a matter of preference. Our group plays 5-6 hour sessions, but for new players, try to go for 3, maybe 4 hours. Start with 3, and keep going if they're still interested.
Online: Gimp with a 'Fog of War' layer that you erase as needed.
While the map packs are classic, a chessex battlemat and some dry erase markers can give any terrain your mind can dream up. They are worth the investment.
https://www.amazon.com/Chessex-Role-Playing-Play-Mat/dp/B0015IQO2O