This is my humble opinion with your new SR, and might I add, congrats!
First off, good insulation is a must! And, when you paint, I recommend selecting a really light color or neutral paint.
Like some of the others recommended, I would do away with an overhead door and install a regular door, and possibly some windows. If you already have an entryway door, you may not want another one, though, just windows to allow a lot of natural light, plus being able to look out!
I would think about installing a ductless mini split system for heat and air. Go with the largest one possible. It is extremely efficient and works great. My BFF in TX got one for her SR upstairs in the attic (not much insulation), but she undersized it when she figured what she 'thought' she needed. It works great, but does not cool as well at the very end of the long room because of lack of insulation. There are a lot of different brands available, so I recommend reading up on them for comparison. And - many of them may have rebates through your electric company as well as federal and state. That is certainly worth looking into.
As for flooring, especially if you are worried that water may seep in, I would not go with any type of wood flooring. I would go with ceramic or porcelain tile. I love the new plank type ceramic/porcelain tile. You can get it in a wood look if you prefer, and you actually cannot tell that it isn't wood until you run your hand across it. Tile is cooler in the summer to your feet as well. And, it is not that hard to lay tile yourself. I've done it many times.
As for electrical, if you can have a dedicated panel, that would be great! My take is to have those baseboard electrical strips installed, as many as you can around the room as well as other outlets. I hate having to run electrical outlet strips that have those annoying cords everywhere.
And lighting, you cannot get too much lighting IMHO. I recommend LED bulbs or fixtures. My GF has a track system in her SR that is like a box (all around the room) and an X in the center as well. She can move her bulbs around, or add more to it. It is sure worth looking into....
Good luck!