WOW thats quite a list you got there
now I will add this and let others chime in for the rest...
Look up as many threads on diffrent sump types and ask ?s of those sumps there so you get it from the makers mouth or the person who bought it.
You can see allot of working diy examples on Youtube
Sump size: This realy depends on what you are gonna put in the sump, type of filtration, under display space,the overall water of the display, the drainage amount when siphon breaks, the amount of water going into the sump and returning into display.
I typicaly go with @ least 30% of water in display ( 10G for 29G or 20H for 55G)
yes you do have to consider all the space avialable under the display inside the cabinet
I typically use diy bucket trickle tower going into sump, some have flow/refugium partitions where I have dwarf shrimps/ fry.
Turnover rate depends if the return pump is the only means of circulation. around 8x is fine for me.
Drilling glass is easy on tanks using anything 1/8"+ thick glass
let us know more details as you start getting closer on your sump design