Halo Wars is nowhere near EndWar. It's camera angles and gameplay controls are completely different.
You start with a base, and you have to build it up. Everything you purchase is bought with Supplies.
So to be able to buy items, you need several Supply Pads.
A Reactor is something I'm not sure of, but it allows you to purchase things that are alot better and improve infanty and vehicles more.
Then you have to build a Barracks which produces Infantry such as Marines, Spartans, Flamethrowers, and you can upgrade these units so they're better in combat.
For example, you can upgrade the Spartans from using Chain Gun Turrets to Spartan Lasers. And the Marines can not only upgrade to using Rocket Launchers, but they can upgrade to ODST's which crash into the ground in a Pod and immediately get into action wherever you choose them to land.
You'll need Vehicle Depot which'll produce Scorpion Tanks, Rhino Tanks and Wolverines which are all used for their own qualities.
Also there's an Air Pad which produces Hornets, and Vultures etc. They're amazing!
When you play as the Covenant, you can get Scarabs! It's truly amazing.
You say you checked out the Demo for EndWar, there's a Demo for Halo Wars. Check it out, but be warned, the real Game is far better than the Demo.
Your first impressions cant be trusted.
If you're unsure, borrow it off a friend first like I did.