Talking of Flagellants, I would still go after the 6th ed metal ones, with a ragtag collection of armour over and under their well worn robes, unkempt and barefoot. Just the right mix of madness, monkishness and mercenary.
With six different sculpts they even have decent variety. A couple of the sculpts are very easy to convert, and the 6th ed Warrior Priests also fit the unit very well.
At one point they sold cheaply in eBay. Possibly more sought after today, or simply rarer because of time passing.
-Z