It would certainly be interesting to put into juice buckets. I my opinion it certainly couldn't hurt. I use my pomace throughout my entire garden. I've noticed a vast difference in yield and growth since I started.
If you ever think you'll do any larger, buy larger. And always press twice after breaking up the pomace. I've got a #40 that I bought new and sometimes it's a little small for me with a group. I'm happy but I've been considering a bladder press because of my volume.
Dont over think it. Choose your yeast for your end characteristic you're shooting for and go for it. Many a wine maker has driven themselves crazy worrying about temperature. Winemaking is an art cloaked in science, don't forget which one comes first.
Battonage, or stirring the lees, is an optional practice but it helps to introduce the nutrients in the lees back into the wine to help the MLF along. I've made a point to do it and I've skipped it and had the same results. If you have a sluggish MLF, it may help
Barrels do best when always full of wine. You can store them with sulfited water but you will be leeching the oak into water and in effect wasting it. The smaller the barrel, the less time the wine stays in to get the same amount of oak. So you would be rotating batches of wine through the...
With that kind of construction you can double as a bomb shelter! Solid plan to encase it in concrete. Might as well build it the right way the first time. Have you considered ventilation as well?