Essentials is a Bukkit plugin, which is for the custom-written Minecraft server called CraftBukkit. FTB is a modification of the normal Minecraft server. So they're completely different pieces of software. FTB doesn't have the custom code necessary to load and understand Bukkit plugins, and CraftBukkit isn't compatible with the type of mods FTB uses.
However! You're not the first person to want the best of both worlds and want to use both Forge mods and Bukkit plugins. Enter MCPC+, which is a further modification of the CraftBukkit server to add Forge, and can therefore accept both Forge mods and Bukkit plugsins.
Alternatively, if you are not interested in the huge constellation of Bukkit plugins and really just want the functionality of Essentials, you can take a look at a Forge mod dedicated to bringing the features of the Essentials plugin to Forge as a native Forge mod: ForgeEssentials.
The advantage to using MCPC+ is that it has native support for both Bukkit plugins and Forge mods. It's also fairly mature, so it's going to be more-or-less reliable (as much as anything is in the world of Minecraft modding). The disadvantage is that every time either CraftBukkit or Forge makes a major release, MCPC+ has to be updated as well so that it is compatible with new plugins/mods, and that takes time. For example, right now it's still on Minecraft 1.5, though for the current version of FTB Unleashed that's sufficiently up-to-date.
The advantage to using ForgeEssentials is that it is a native Forge mod, so it is simpler to install than dealing with two modding flavours and it will more quickly track updates to Forge and not stay out of date as long as MCPC+ may potentially do. The disadvantage is that it only adds Essentials-like functionality and doesn't give you access to the wider world of Bukkit plugins (though I hear that's a long-term goal). It's also still under heavy development, so it's not the complete, awesome, bug-free package that the Bukkit Essentials plugin is, but it might already offer the specific features that you're interested in.
Due to ForgeEssentials' WIP status you will probably want to go for the bigger-but-better MCPC+, but it's good to have options.
WATERMILLS
Although watermills are fairly cheap to make they provide very little power. I am fairly certain of this assumption since you are talking about a 'watermill powerplant' that your intention is to use multiple watermills to power your desires. In the case of owning multiple watermills, you probably wont want to fill them manually since that is way to time consuming so i'm guessing you go for the automatic (unmanned) method.
Unmanned: 0.010 EU/t per block of adjacent water. The maximum output of an unmanned Water Mill is 0.25EU/t (3x3x3 cube of water minus the mill in the middle, and 1 piece of cable to direct the energy = 25 water tiles).
As stated on the FTB wiki a single unmanned watermill surrounded by the maximum amount of water blocks can provide 0.25EU/t. Since the watermills have no internal power buffer it is suggested that you feed the watermills into some sort of power storage device (the cheapest one is being the Batbox).
Also stated on the wiki is the fact that the Batbox can accept a maximum input of 32EU/t. This is also the maximum amount it can output.
Short side-note: Your machines will soon start to require more than 32EU/t so even if you should decide to get a lot of watermills it wont last to long.
If you decide to go for the eco-friendly method of using only water power you need (besides a lot of space) a total of 32EU / 0.25EU = 128 water mills. Yeah that are a lot of water mills (also not so cheap anymore). Although its a very time-consuming, expensive and not the most effective method of gaining power, it is a viable one and so you could decide to choose this.
GENERATORS
An alternative to using watermills are a few generators. Now i don't have hand-on experience with your mod-pack but a quick google search showed me that the modpack contains Steve's carts 2. This means that you could fully automate a tree-farm providing you with unlimited wood (which you can obviously use to power your generators). Although this method does require a decent amount of minerals to start with its not that bad.
How do i build a tree-farm then?
Generators provide a maximum of 10EU/t so four generators are enough to power your early needs. Upgrading your tree-farm also means you can upgrade the amount of generators and so increasing your power output.
AQUEOUS ACCUMULATOR + STEAM ENGINES
and many many more methodes. You'll be surprised how much you can learn by simply googling.
Best Answer
Quarries run on Minecraft Joules, or MJ.
RF converts to MJ at a rate of 10:1, so 80RF is 8 MJ. Also, you don't need a tesseract to convert RF to MJ, you just need an energy conduit that is connected to a MJ consuming machine.
As for how many dynamos you need to power a quarry, quarries take up to 100MJ per tick (MJ/t), but will run perfectly fine on 25MJ/t.
To produce 100MJ/t, you will need 13 dynamos, which will be capable of producing a total of 104MJ/t (1040RF/t). They'll auto-throttle to produce only as much as is requested, so one will likely only run at 4MJ/t (40RF/t).