I'm on a similar mission, but with the advantage of having a got a lot of them from when they came out, but with gaps here and there. From 4th edition onwards, the only stuff I'm missing are the Estalian style handgunners and swordsmen, a couple of the Horse Archer variants, one of the Mordheim Witch Hunters, which is still available from GW online, but you can't pick which if the 4 you get given, and I don't want to buy 10 to get the 1 I want, and end up with duplicates. And some of the rare multipart Sisters of Sigmar variants. I think I have pretty much everything else from 4th onwards. And quite a lot of the Marauder Miniatures stuff (Which back then would have been used by anyone no-complaints, unlike the is it official or not arguament you get with Forgeworld these days). And I've got lots of the Citadel Fighter series, some of which was labeled as Empire towards the end of 3rd Edition. And a handful of pre-slotta stuff which really aren't worth chasing down unless you're truly obsessed and minted.
Of course I'm saying this to illustrate what I've collected, and to show off a little bit,but let me be clear, I'm not selling! Sorry!