Using wooden posts I take it?
They will be already pressure treated, so no painting required. Best thing to do is build the concrete up, slightly above the normal ground level. You can chamfer it off to make it look nicer. This will help keep the base dry, it won't sit in the wet ground all the time and last three times longer
You will only be able to do 1 bay at a time if you use the castle stuff as it goes off really quick.
Dig all your holes then used a piece of timber cut the same width as your panels to precisely align the posts when you put the concrete in. By the time you've got them all in, you will be able to get straight on with the panels.