If I may join the discussion
. Groblog is a must since you don't only save 1 Might per shaman, but you also gain Groblog's Might points. If your 500 point army includes 3 shaman, this amounts to quite the extra Might points.
Furthermore a frontline of Gundabads supported by regular Warriors is an excellent idea; coupled with the 5+ save they become all the more durable.
Giant bats are tricky to use (without including Druzhag). I would suggest for the 500 points level use a Cave Troll or a Dweller in the Dark.
Durburz has a very good store of Might, and will hold your line in case something goes awry with the Shamans; he is good for 500 points games.
I would never include a drum in under 650-700 point armies.
With the new rules Goblin archers are - in my experience at least- horrendous.
Overall Moria is a diverse faction, and I'd suggest waiting for the rulebook before deciding on a list, since there are many different, yet equally viable ways of building a Moria force.
An example 500 force that I would happily take is:
Moria Goblin Shaman (123 points)
6 Gundabads with Shields
6 Moria Goblins with Spears
Moria Goblin Shaman (123 points)
6 Gundabads with Shields
6 Moria Goblins with Spears
Moria Goblin Shaman (123 points)
6 Gundabads with Shields
6 Moria Goblins with Spears
Groblog (130 points)
Dweller in the Dark
41 models
6 Might