Right, I've fleshed out and made plenty of NPC's and Enemies in my time, but I have a concept for a group of Dark Side Adepts which would fit really well with my campaign - the problem being I have NO IDEA where to start.

They need to be combat effective and use a lightsaber, no other force powers beyond that. They also would have around 250-300xp to make them deadly enough for my players. Any advice or tips?

Depends on how Combat oriented your players are. In my case, my players are very strong in the Man-handling of enemies. If they are a combat group, I would suggest putting 3 ranks into Lightsaber, and distributing points in Athletics, Coordination, Discipline. You say you don't want Force Powers, but a really good one to use is Force Enhance, if you look at the Force Leap ability you can really have them get the jump on your players.

Two are Melee oriented, one is a Ranged expert and the other two are mediocre at both.

I did look at at that, but really the only combat oriented FP I can see would be Move because of the option of hitting your opponent with it, hence why I wanted them to just be lightsaber trained and not much else.

Well Enhance also lets you change the game around, cause you can also commit a force die to increase your Brawn or Agility by 1 for the remainder of the encounter. Move is the most powerful Force power in the game. If you can pull it off at the 42% chance of getting a light sided point, and getting I think 2? maybe 3.. You can throw a Silhouette 4 Starship at someone at EXTREME range, doing 40 Damage instantly. Very powerful. The other abilities are very fun. There was also the "Way of the Force" unofficial supplement. It contained a Dark Side Acolyte as well as Force Injure, which when capped can do 5 Damage that ignores Soak to up to 3 targets...including droids.

That could be fun
As for "Way of The Force", Will a quick Google bring that up or is there somewhere you could point me?

EDIT - Found it! Seems like exactly the kind of document I was looking for! Thank you very much, my players will hate me

The only issue I can foresee with Way of the Force is the Reflect Blasters ability with the Jedi Initiate. I think it is a bit much to be fair, but who am I to say.

I had been thinking about something similar mostly due to the gm of my old WEG Star Wars game always resorting to using Darth Vader as well as having Luke fall to the darkside at the end of Empires.

What I've been tinkering with was two orders one called the Brotherhood which involved male force users recruited from the sensitives captured by the Empire who tend towards the Darth Maul style of combat but aren't that strong with the force and the Sisterhood which usually consists of a trio of female force users specialising in one aspect of the Force (Telekinesis, Precognition and Telepathy being one idea) but are backed up by a group of Stormtroopers as their combat skills are their achilles heel.

The Brotherhood usually operate on their own and run schemes designed to weaken the Rebel Alliance and the Sisterhood tend towards espionage and information brokering trying to discover the weak points of discovered enemies often leading to rebel cells being rooted out with frightening ease.

Will have to give that supplement you mentioned a look.

Without giving too much away (I Know at least one of my players is also a GM and frequents these forums), my party has stumbled upon a secret Imperial Bio Weapons Research plot, and they're on their way to another facility to thwart the plans there.

My plan for one of the facilities is a Shadowtrooper-esque training facility, which is where I need the Dark Side force user "template". The Ways of The Force is a fantastic supplement and will tide me over beautifully until Force And Destiny Beta is released.

Well then, I'm glad we could help! I use every and any supplement I can find for EotE / AoR for my game. I really like how WotF deals with construction / repair of a lightsaber.