Great advice above....

to recap my experience...

1. you must cut another door for safety and the locking doors in the wind thing, yes a long shot but you are screwed in a fire.

2. A flux core or MIG welder is best...dont stick weld them.

3. The weigh a lot, about 6000 lbs for a 40 foot one and thats with minimal outfitting. Moving is not trivial.

4. My design includes a mud room on the big door end. I build a wall several feet inside and put a door in that wall. Then we pin the steel doors back all weekend. The mud room is where we lock up the generator and anything else we leave there including a ATV.

5. Seal up the gap underneath from snakes etc.

6. Dont go crazy with insulation, say R5 is plenty. It is a small space and heating it up is easy. More insulation than that is a waste.

7. They are sucure if you put plates or bars on the windows while away. This is a big issue in South Texas. I perfer plates on windows and bars over doors.

8. Twenty foot units are fine for a kids rooms or storage but its a waste of time as a cabin unless you are solo or you and missus...

9. I like flourescent lights, since they are cool...not generating a lot of heat.

10. One window unit in each end seems to do the trick. Size depends on where you are....bigger in far south texas or smaller in far west...

11. cost to finish out is in the eye of the beholder. some guys spend weeks in one notch better than cardboard shacks and are happy. Some want a sofa and sat dish. You can do anything to a container.

12. buy a 40 foot if primary quarters and get the tall one, for the extra foot of head room. Well worth it.

I can't remember what I last saw with the link to the home I saw. They took 2 of the 40' ones and welded them side by side, cut an archway between the two and had bedrooms, dining area bath and kitchen, windows, doors etc. The pushed the whole thing into the side of a hill and it was awesome. I just don't have the welding skills or I'd be looking at doing it myself. If I find the link I'll post it.

how do you insulate ?

I just bought one of these to live in while building a home. I bought the Angler 14'x32'. Had it made with no porch. Very good quality and portable. Delivered and setup it was around $24,000. You can custom design your own floor plan also.
http://www.morgancabins.com/

Most folks use spray foam insulation in container builds, doesn't take much to make a huge difference.

Justin shubert in Graham is the biggest dealer in north Texas, delivers all over the state

Justin is a good guy to deal with and his driver can place a container just about anywhere you want one.

Out in the desert in California people are making houses out of these things. Like a modular home.

like this one?

Shipping Container Home - Imgur Album

My cousin has contracted out to bury them for "preppers" who want to shelter underground...

A lot of people make shipping containers into homes these days.

Mostly hippies and Longhorns, but it's done pretty regularly.