I've had a look for you, and both the Screaming Bell and Plague Furnace, on pages 42 and 48 of the Skaven army book, say:
"The [Bell/Furnace] is a special kind of mount for a [Grey Seer/Plague Priest]. [It is] treated as a single model with combined characteristics for the [Bell/Furnace] and its crew."
For Initiative, that makes it easy. You use the highest Initiative of the crew, so Purple Sun is easy to resolve.
Leadership for Spirit Leech is tricky. They are listed as "-" because both constructs are Unbreakable, and thus would normally ignore leadership entirely, but not so in this case. As a mount, you would be able to target the construct, but it has no Leadership value.
Until it gets FAQ'd, I'm sorry to say it would have a Leadership of 0, therefore Spirit leech would automatically cause at least 3 wounds with every application (Ld 0+D6 vs. Ld 8 or 9 +D6 means you will definitely win by between 3 and 14!)!!! Bear in mind though that the Screaming Bell gets a 4+ ward save. Congratulations, an Elf army has won against Skaven before the game has even started. As usual.