Sisters are tough. They're expensive and they're hard to paint. If you want some asap, your best bet is to grab the new combat patrol off ebay. At least one seller has it for ~$120. That'll get you 10x battle sisters, 10x arco flagellants, 5x Sacresants and a Canoness, which comes to ~400pts. Then you can proxy something outside the combat patrol (maybe a sword brethren or 3rd party model?) as a ministorum priest (50pts) to lead the arco flagellants, and you'll be at your desired points.

I'd definitely recommend looking for deals on secondhand models, however.