You can use the Space Hulk Calistarius (Librarian) model for Mephiston -- It doesn't look like the Mephiston model, but it *IS* Mephiston, before he becomes Chief Librarian. Personally, I hate the Mephiston model; I think it's ugly.
Corbulo, use a Sanguinary Priest. Not only is Corbulo a sanguinary priest anyhow, the models are very similar (cup and chainsword), except the priest is way better.
Dante, as you say, is easy. Use Sanguinary Guard bits + Death Company jump pack. There's a kitbash in my gallery you can see of him.
Dunno why you want Tycho

But if you really want him, use bits from the
BA tac squad and death company.
Astorath and Sanguinor actually have nice finecast kits. However, they are not very good models to play with, so I don't know why you'd bother. But you could use sanguinary guard for Sanguinor, and make scrolly things out of greenstuff. Astorath's wings look like Sanguinary Guard wings (Sort of, or you can use scourge wings), but his body is highly unique.
Crassor is just a Furioso kit (using the
DC build).
I don't know anyone who plays Gabriel Seth as the
LoW over Dante.

But if you really want him, it's just use a space marine with a chainsword and some funky backpack banner thing
Karlaen is plastic, so yay!