I'm currently converting shipping containers (20' and 40') into dog kennels. I've timber framed the interiors because they're going to be buried. When buried, the sides of a regular container deflect inward about 4" for an 8' container and about 6" on a hi-cube (9'6" high) unless supported (wanna see pics ? lol). That is enough to crack the paint and put groundwater in contact with the steel. I'm using foundation damproofing on the outside of the steel - it's a thicker film and more flexible than paint and it's made to be buried. Man, do they ever warm up in the sun between painting and backfilling though !
Ventilation is a must, obviously.. I've buried one right to it's end and the insulation value is pretty impressive : 20-odd dogs barking inside is barely audible unless you're facing the (unburied) door, and then only because I haven't finished insulating it yet.
I don't recall how the math worked out on air changes (check your local building code), but I'm using Broan DX90 bathroom exhaust fans to change the air.
Insulation is required only where the steel is exposed to the air: the temperature difference will cause condensation.
To live in it, I'd definitely figure out a way to get some natural light inside. I found when I was doing the final hookups I felt like I was in some kind of a cell after a while and I'd go find something to do outside...but maybe that's just me lol. There are skylights made for use in a trussed roof that have an internally mirrored duct to bring light through an attic. Maybe a couple of those ?
As far as $$ goes, our containers run about 2500-3500 (used) and about 400 (I think) to get them shipped about 100km (including a ferry at about 100 bucks).
One thing to check is permits. I need permits only to have the containers on site. Both of the local building inspectors have been pretty lax because there's no real precedent to come up with building standards and (as mentioned) it's a different loadpath than conventional construction. I actually had the second container's framing inspection signed off over the phone and email pictures after he'd seen the first one

I expect the rest to go the same way although he does drop in from time to time, probably out of personal interest.
I'm planning to do this myself as a cabin's basement, so I bid this job to get someone else to pay for the learning curve. If I can share some of it so much the better. I'll try and get some pics up in the next couple days.
Final thought: It sounds like a good idea (but), you might be best to avoid any external mods before the unit is onsite. Confirm with the applicable transportation regs for your jurisdiction, but any structural mods have to be engineered to be legal for transport. No point paying for that if it's only going for one more haul...
EDIT: just a thought guys, but I assumed he would put at least as much diligence as he's showing here into choosing the right piece of dirt...
Good point on sewer sections for the tunnel. (I didn't know that about the snow 'path'). Sewer pipes by their nature are sealed as new sections are applied. A threaded cap (the biggest I've seen is 36") with a handle on the inside might be the answer for the 'outflow' end.