If I want to get all the Descent 1E expansion figures, what Runebound packs will I need to buy?
If I'm not mistaken, you already own the D1E core box, based on conversation in another thread. That being the case, you have no use for Runebound core or the Island of Dread expansion. You also have no use for Runewars or its only expansion, Banners of War. (Unless you want them as games in their own right, of course. I rather like all the Terrinoth games.)
You can get 6 new heroes in Runebound: Sands of Al-Kalim. The same 6 heroes come in D1E: Altar of Despair.
6 new heroes in Runebound: Frozen Wastes, which are the same 6 found in D1E: Tomb of Ice.
6 new heroes in Drakon (3rd edition), which are the same 6 found in D1E: Well of Darkness.
Finally, 6 new heroes in DungeonQuest, not available elsewhere.
Plus 4 "promo" heroes that are notoriously difficult to find. (You can probably get Nara the Fang in a copy of D1E: Sea of Blood. She was technically only included in first print run copies, but I'm not sure how many additional print runs were actually made - it seems like every copy I've seen had her in it.)
The 6 heroes in Runebound: Mists of Zanaga were never translated to D1E and therefore do not have hero cards in the D2E Conversion Kit.
Note that in terms of monster figures, there really isn't any substitute for the D1E expansions. Runewars monsters are a different scale so they don't really work as sub-ins for their Descent counterparts. Besides which I think that any Runewars monsters you might hope to recycle are already represented in the D1E core box anyway.
For monsters, you may also want to keep your eye on BattleLore 2.0. I doubt any of the figures in the base box will apply, as the two main factions are Daqan (humans) and Uthuk Y'llan (who are thus far not well represent at all in Descent, 1E or 2E.) However, future expansion material might prove to give you something to work with. Only time will tell.