Questions from a new builder

Hey guys! So I am a first time TD builder and I have a few questions. I have been trolling the forum for the past few weeks and theres a few questions that I haven't been able to find answers too.
1. I am building a cubby 5x8 on a northern tool trailer. I will not be skinning the camper. My plan has been to construct my walls with an outer sheet of plywood, spars with foam in-between, and an inner sheet. What thickness plywood do you guys recommend for the inner and outer sheets? 1/4" or 1/2"? As well, are 3/4" spars sufficient for the walls and roof? or should I use 1"?
2. For the roof, what thickness plywood should I use for the outer layer(s)? There is no Baltic birch in my area. But there is 5.2mm White birch. Is that comparable to Baltic birch? Also, is 5.2mm the same as 1/4" or is It a little thinner? Or should I go the route of 1/8" luan? If I did use luan, is 1/8" sufficient or do I need to use 2 layers if 1/8" since I wont be skinning it?
3. What process would you recommend for sealing/painting the TD?
Sorry for all of the questions. I appreciate any input that you guys have for me!
1. I am building a cubby 5x8 on a northern tool trailer. I will not be skinning the camper. My plan has been to construct my walls with an outer sheet of plywood, spars with foam in-between, and an inner sheet. What thickness plywood do you guys recommend for the inner and outer sheets? 1/4" or 1/2"? As well, are 3/4" spars sufficient for the walls and roof? or should I use 1"?
2. For the roof, what thickness plywood should I use for the outer layer(s)? There is no Baltic birch in my area. But there is 5.2mm White birch. Is that comparable to Baltic birch? Also, is 5.2mm the same as 1/4" or is It a little thinner? Or should I go the route of 1/8" luan? If I did use luan, is 1/8" sufficient or do I need to use 2 layers if 1/8" since I wont be skinning it?
3. What process would you recommend for sealing/painting the TD?
Sorry for all of the questions. I appreciate any input that you guys have for me!