A stronger Finnish Navy before the Winter War

Would Stalin still attack Finland if the Finnish Navy had proper warships before World War II?

Probably; Stalin would likely still order the attack, but stronger Finnish naval resistance would make the Soviet advance even less successful.

A better Finnish fleet would not necessarily deter Stalin from attacking. The decision to invade is treated as strategically unsound enough that additional Finnish naval strength would probably not change it. Stronger resistance would instead make the existing Soviet failure worse and restrict the advance further. The counterfactual changes the invasion's results more readily than the decision to begin it.