Hi,
Isengard is a great faction to play. They have F4 for all warriors (uruks, at least), and cheap heroes (but lack of F6). S4 for warriors and S5 for heroes is great too!
Saruman is a great choice but he's expensive, you won't field him in lower points games... When you do, include Grima if you can afford, he's a real pain to play against !
Lurtz and Ugluk aren't really powerfull (lurtz has no special rule, and because of his bow, he can't benefits from the shield... he's only D5... - Ugluk special rule is quite situational).
I suggest you taking Vrasku (crossbow sniper ^^). Mauhur is powerful with his 3 attacks, increased speed, but he only has 2 might... Captains with heavy armor and shield are D7, which is pretty good.
Uruk-hai Shaman are pretty good too, with their "Fury" spell!
For the warriors, scouts may be useful for mobility with their increased speed if taken with Mauhur. Otherwise, go for Uruk-hai warriors (best defense). You can make pike blocks (1 shied and 2 pikes behind). If you need shooting weapons, crossbows are deadly (S4, hits on 4+...). You need to quickly go to a strategic point, however, to make them effective. And when they are not useful anymore, send them to battle. With F4, S4, D5, they'll still do some damage ^^
You also can add a banner if you want to, this can help with pike blocks.
Berserkers are killers, I always include a few in my armies and send them against the threats or to kill infantry
Ferals Uruk, well, I've just received 2 so I'll include them in my next game :pPay attention not to loose those elites before the armies clash. I tend to keep them safe until that moment (they'll be pincushion every game if you don't) :p
I've never used Dunlendings, or orcs, or Isengard trolls, so I can't help you with these units.
If you need mobility, wargs are cheap but can cause havoc against low def units. They have weak defense (D5 with shield), so don't let them be fired at! Use Sharku to lead a few amount (5-6 wargs), and keep them together (cavalry need to move first, heroic charge with Sharku and his 3 might points !)
I hope that helps you, and sorry for the bad English