ALT world & Mega Pencil - what does my money get me?

The Alt World:

Alt World is a repeat of all of the stages in the standard world, but with a twist. These puzzles are done a bit differently. All the puzzles in the standard world are duplicated so you can get Pokémon here if you haven't yet got them in the main world. Each stage also has its own set of challenges and Picrite rewards for completing them.
However, while the Pokémon in each area is the same between worlds, the placement and order of them varies.

Regarding Picrites:

You have to spend 300 Picrites to get to the Alt World and 500 Picrites to get the Mega Pencil to complete Mega Evolved Pokémon stages.

So you have to buy the Mega Pencil just once.

And about micro-transactions:

You can only purchase up to 5,000 Picrites in the lifetime of the game. Once you have done this, however, you'll be able to get free Picrites. You'll then be given them in batches of 1,000 until you hit 8,000 in your pack preventing you from running out of them at any time in the future game play.

Source Serebii.