If it were me, I'd avoid used pistons just on the basis of, you're spending all this money machining the block for what is going to be a really sweet setup, if you slap worn pistons in, it's going to negate the benefit of that freshly machined bore.
I usually get pistons from KG Trimning, but I appreciate a set of four is nearly $1000.
https://www.kgtrimning.org/200-ser-1/engineparts/engine-block/piston/3735.html
NOS pistons could be a great choice, just be sure of their condition, if they were stored improperly, maybe in a humid environment or maybe the gudgeon pins weren't kept in their wax paper wrappers, you might have surface defects, corrosion, etc, which will either negate the savings or just render them useless. Replacing the rings isn't the end of the world, but I don't know if you can even buy gudgeon pins by themselves.
