The resulting artifact creature will be able to attack if it’s been under your control continuously since the turn began. That is, it doesn’t matter how long it’s been a creature, just how long it’s been on the battlefield.

If Synthesizer Labship’s second ability causes a Vehicle to become an artifact creature, it doesn’t count as “crewing” that Vehicle for any ability that would trigger due to a Vehicle becoming crewed.

If the target of Synthesizer Labship’s second ability was already a creature, its base power and toughness will become 2/2 until end of turn. This overwrites any previous effects that set its base power and/or toughness to specific values. Any power- or toughness-setting effects that start to apply after Synthesizer Labship’s second ability resolves will overwrite this effect. Effects that otherwise modify the target’s power and toughness, including +1/+1 counters, will still apply no matter when they took effect.

If Synthesizer Labship’s second ability causes a Vehicle to become an artifact creature, its base power and toughness will be set to 2/2. Crewing that Vehicle will not restore its power and toughness. Similarly, if the ability causes a permanent with station to become an artifact creature, putting an amount of charge counters on it greater than or equal to the amount required for it to become an artifact creature won’t overwrite the base power and toughness set by Synthesizer Labship’s second ability.