Hello there!
Im new at the game aswell, but can maybe help you a little!
I would recommend you to watch at GBHL (
https://www.youtube.com/user/GBHLPodcast) and DCHL (
https://www.youtube.com/channel/UC9Eja6 ... eAyLyTpJsA)
For some inspiration and to see all the different kinds of armies.
Like you mention it can be good to buy one Good and one Evil army.
As the army you want. It´s all your choise and the best way to decide is to get as much informationa s you can! The GBHL and DCHL can help but maybe also some Scourcebooks.
There you can see all the choises you have for each faction. "Rohan", "Isengard", "Arnor", "Mordor" etc.
They can be bought on GW site, just "Mordor" and "Moria and Angmar" still there tho, The rest can maybe be bought on Ebay or some bookshop if you are lucky!
As you have bought some models from Ebay. Continue with them, You got 14 Uruk´s then buy maybe a Captain and you got one Warband done.
You can only lead models Models in your Faction list.
If you find a hero in the list, he can lead a warband. If he is not, you can take him as a seperate hero or as a warband leader with warriors from the list he is listed!
Just think like this: Is he in the Rohan list;
Yes= Then he can lead warriors from the Rohan list.
No= Then he can´t lead warriors from the Rohan list
Hopefully this helped a bit.
Happy New year!