I bought a house built in 1989 with Andersen casement windows. One window has this wooden bottom frame missing. Do they sell these? Can't seem to find anything online, and the screen won't fit without it.

I've got an interesting situation. I'm installing windows in a what was a screened in room. I've torn out the old screens and wood trim. The walls are constructed of (from outside in) WP-4 siding, 1in foam insulation, 2x4, WP-4 siding. The trim on the inside and outside is 1x2 or 1x4 on top of the WP-4. This all adds up to about 6in (without the trim). There is no moisture barrier. Pictures are attached. I have ordered windows without nailing fins - Andersen 100 series windows. I've read through countless installation instructions, and I think I am close to the best solution. My current plan is to cover the entire window perimeter in butyl tape, overlapping the inside and outside by an inch or so. Then I would install the windows, caulk the outside and foam insulate/trim the inside. This seems like the best way to maintain some level of water tight seal around the window and the siding. Does this make sense? I have another window that has been replaced and I am not quite sure how it was installed and sealed. I've attached a picture of this to give an idea of how the rest of the trim on the house looks. Any suggestions/advice/comments would be appreciated.