So I wouldn't grab Amdur just yet unless you plan on using him instead of Khamul (which might be a good idea anyways)- you ideally want to have 12 warriors per hero as that forms a warband. Now if you have the old Easterling command, all you have is a bearer and a captain but if you have the new one you also have the dragon knight and easterling which are both excellent heroes. The Dragon Knight cannot lead troops so ideally you will have
Khamul + 12 guys
War Priest + 12 guys
Captain + 12 guys
Dragon Knight
Since you only have 24 warriors, you could either buy 12 more foot soldiers. Now this isn't strictly how you need to make your army, you could always buy kataphracts (which come with a mounted captain) and now have your captain with 5 other kataphracts or you could skip the captain bits and give Khamul (who im assuming has a mount) 6 kataphracts to lead, either way is good. Personally I recomend the kataphracts as they come with another banner, which you could also convert onto a foot soldier if you wanted.
After you buy foot troops or cavalry, I would ideally buy the mounted dragon knight but its pretty expensive so maybe not. You could always convert another foot soldier into another captain, give him foot soldiers, or you could take Amdur and give him foot soldiers but this is more expensive. Basically just any time you take a hero other than a dragon knight try to give them 12 warriors or at least 5 cavalry.
Another option is take some Monster, either taking a ringwraith on a fellbeast (count as khamul/convert), spider queen, or even a troll.
Also I would definitely recommend buying your fallen empires book and the mini rulebook, however for models outside of those you can consult this army builder:
http://lonelyknight.0fees.net/ It doesn't mention starting wargear so it really cannot replace buying the army book but its incredibly useful. Have fun!