Richard (et. al.)
If you want an install without any Internet connectivity - you have one really good choice - but you may need someone to provide you with media made on another Debian machine.
The jigdo file which produces a 16GB file for writing to a USB stick is, essentially, the first three DVDs plus a bit on one medium. It's directly bootable on a machine that supports boot from USB. it will boot in legacy (non-UEFI) and UEFI mode. It's ideal. The one thing it doesn't contain is firmware - but that can be written to another USB stick.One small, cheap, old stick, one newer stick and some internet connectivity _somewhere_ and you can do it.
That way, everything is met. I have asked Sledge if he would be prepared to produce _another_ non-free image in 16GB size but he replied that it wasn't particularly worth the increased bother and storage size of maintaining the 16GB file for every point release when it could readily be regenerated.
The alternatives are the BluRay media (or possibly the debian-edu media) both of which are around the same size.
This isn't rocket science - but yours is a distinct edge case. Installs to a partition and using the partition to bootstrap a second install are likely to be tested by only one person - yourself - and you would probably need to submit very detailed bug reports and a significantly compelling use case to achieve major changes.
All the very best, as ever,
Andy C