I'd talk to your powdercoater about the blast media and most of the plugging questions. Have they done bikes before? If not, you'll need to make sure they mask the seat tube, fork crown race, etc or you will need to ream and machine all those spots.

In general, I think they just plug up most of the threaded bits. If you need the BB shell or head tube faced, do it afterwards. You might not need to do it all but if you do need it then afterwards is better.

No framesaver beforehand!. The powder coat is finished by baking it in an oven and it can't be good to have a frame full of oil for that.