Wood for Wendy House

[North]

Hi I am planning to start building a Wendy house for the little one.
I have treated timber for the frame and decking for the floor but would like to know what would be the best and cheaper wood for the exterior bearing in mind that I would like to paint it in a bright colour i.e. sky blue/green etc
Any advice would be welcome
Thanks

 

[Nozzle]

A stash of old pallets which is all sanded back to get rid of the splinters.

Nozzle

 

[big-all]

cheaper exterior ply or osb/sterling board 3 or 4 years

5" pressure treated txg shiplap 7 years plus 3 times the cost