Is Goku's base form his final form?

It would eventually be trumped by Super Saiyan God and Super Saiyan Blue in Dragon Ball Super, but in the context of Dragon Ball Z, it was Goku's "final" form. The Super Saiyan 3 form multiples a Saiyan's base form power times 400, though at the cost of draining stamina and ki reserves at an alarming rate.

Is Goku planetary base form?

Technically, he is. Remember, Goku has defeated characters that have demonstrated Planet-Busting levels of power, such as Vegeta, Frieza, and Kid Buu. Believing that Goku isn't planetary at bare minimum is simply ludicrous.

What is Goku's base form?

Let's start with the basic form. Goku, in his base form, is still a force to be reckoned with. He spent all of Dragon Ball, after all, in this form (with a couple of noticeable exceptions). However, in base form, he is at his lowest level of power.

Can Goku destroy the planet?

So yes. Goku can do it without effort. Everyone in Dragonball series is restraining themselves to avoid the destruction of their surroundings or even the planet they are standing on. He can destroy any planet only by raising his power level, he did not even need to punch at planet.