Exterior Trim/ Flashing
I managed to find 1x3 pieces they call "furring strips" which aren't that expensive and paint and prime up nicely. We use "swiss coffee" for all of our neutral white-ish needs because it is pre-mixed at Home Depot (i.e. saves 10 min per trip). I cut these and attached them all around the exterior to gussy up the place a bit and help close the air leaks at the door and window and am happy with it. I measured the corners and attached them to each other before attaching them to the greenhouse, which helps the corners look more closed.
For the tops and sides, I got metal flashing painted white. I installed it with roofing screws just under the roof glazing (and over the trim) on both sides and over the trim and flashing in the back (yes, I neglected my photographic duties here).
Fingers crossed for me for the winter that this keeps the rain out!!
We decided to use thick stryofoam insulation on the interior where the siding is. This turned out to be really messy, with little balls of styrofoam all over. I covered the top edges with silver flashing tape we had left over from a window project so the kids wouldn't pick at it. I left the silver reflective side facing in, and uncovered.
Backerboard (for Tile)
Cement board (aka backer board) for tile comes in 3x5 pieces, so we got two of the thinner variety. It cuts nicely with my circular saw as well, although it is really dusty.
[Here is a side note- check out my window opening. You can see the additional stud sideways below it, and the stud keeping it up. That is my kids' step, so they can climb out the window without breaking anything- and end up on the ladder.]
We put in the backerboard and used yellow drywall tape at the seams.