Tezzeret the Seeker

The effect from the ability overwrites other effects that set power and/or toughness if and only if those effects existed before the ability resolved. It will not overwrite effects that modify power or toughness (whether from a static ability, counters, or a resolved spell or ability), nor will it overwrite effects that set power and toughness which come into existence after the ability resolves. Effects that switch the creature’s power and toughness are always applied after any other power or toughness changing effects, including this one, regardless of the order in which they are created.

A noncreature permanent that turns into a creature is subject to the “summoning sickness” rule: It can only attack, and its tap abilities can only be activated, if its controller has continuously controlled that permanent since the beginning of their most recent turn.

For the second ability, you choose the value of X when you activate it. You don’t look through your library until the ability resolves. (In other words, you can’t look through your library, decide what artifact card you want, and then determine what X is.) You can’t choose an X that’s greater than the number of loyalty counters on Tezzeret.