There are some minor differences in building techniques to consider if you're going to hire a contractor to finish it out on site.
I've done about a half a dozen for a company in Carlsbad, NM.
1. Water intrusion is your biggest enemy. Welding flat stock metal into all openings before windows and doors are installed is the biggest factor I learned. I originally just installed flashings and caulked them (with corresponding tabs to accommodate for water flow.
2. Additional measures to account for standing water on the top of the conex should be considered. Conex's are known to have standing water sit in areas in between the corrugated ridges on and consequently, over time they can rust through. Not an issue for you if you're going to sell soon, but may want to consider that if you live in a wet climate and will be responsible for long term maintenance. Elastomeric coatings are used frequently out here (in NM) . It comes in a paint bucket and can be applied with a brush and roller or sprayed on (you need a specialty sprayer don't run it through your typical household sprayer). You'll want to coat it every couple of years but that is the most economical option in my opinion.
3. Consider how you're going to build out the interior walls. If you're going install drywall or paneling, wood 2x4's will accomodate plumbing and electrical. If you use metal framing or something different your electrical will need to be encased in some sort of conduit (shielded armor is a flexible version that should work). Of course that's assuming you're pulling permits.
4. The ones we built were essential break rooms and field offices intended to be semi permanent. We used wood paneling (3/8" T1-11) on the interior. It's much more durable than drywall as the interiors would potentially taking a lot of abuse (can't put your boot through wood paneling). As such we the rough openings for doors and windows we built so that we could remove the windows/doors without having to remove the wall paneling (Think drywall around a window that would be cut out in order to remove and replace a window.). In the event a window pane was broken, we would be able to come in, remove the trim, unscrew the window, install new and then replace trim without cutting out wall paneling and replacing.
5. Considered heating and cooling unit placement. 1st consideration to this is egress. If you install a window unit then another window must be free for someone to exit in the event of a fire. Second is if you are installing in the side of the structure, consider not installing near a pathway where someone will hit their head or a car door will hit it when parked.
There's a couple companies that I know of (and a TON more that I don't) that sell pre fabbed versions.
Xcaliber out of Graham, TX and a new one that is starting up out of Las Cruces, NM is underbox USA.
If ya'll have questions you can message me here on BP