I had a good experience with PMB performance who restored my 400 calipers for about $220ea. From this: Image Unavailable, Please Login
Like a dentist showing you the xray they return your old pistons and highlight the corroded areas. They also cleaned up and coated the pins and retainers at no extra charge.
These guys look like they really know what they're doing. You're gonna have a lot of pride putting those back on the car!
Very wise choice! I've got a caliper restoration nightmare I'm still trying to recover from. You did very - VERY well!