Cliffs notes:
This is doable, but make some concessions to safety and your family. I only spent ten minutes typing this because I felt like you have a good idea that needs tweaking, now go make it happen. There are a lot of guys on here who are giving you valuable professional advice, as well as the benefit of years of experience. Don't be so quick to dismiss that, it's a mistake. All suggestions are made in the spirit of helping you out, not antagonizing you.
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Put up the wall now, while you can integrate it into the rest of the building project- it'll cost less. I guarantee you you'll eventually be doing it anyway, either to keep the kid out of your tools or because your significant other gets tired of looking at garage stuff all the time. Women can be funny like that. A few hundred bucks added on the front end of this project will yield big dividends in comfort for you and yours.
Since you're doing this as a steel building, are you going to have ceilings on the living space? I gather not, since you're planning on using infrared heat hung from a purlin. I know you're in TX- what about AC? What are your plans there?
I completely understand wanting to go your own way on this, that's fine. Part of being a parent, though, is taking any reasonable precaution to protect them. Wait a year, and that 2-year-old is going to increase in speed 400%, and probably listen to you less. Structure your build now with that in mind.
If you're bent on doing it this way, here would be my completely amateur suggestions/ what I would do:
-Build a curb to prevent fumes/gas spills from getting in the living space- cheap insurance. You can get rid of it via jackhammer later (12 years later) if you need to.
-Build a wall between the shop area and living quarters. If you want it open all the time, fine, spend a little money on a nice set of exterior french doors. That way you can leave them open if you want most of the time, and close them up when you're out there making racket at 1AM and your wife and son want to sleep.
-I'd go ahead and close the building envelope of the living area, just build a box in a box. It'll be cheaper to heat and cool the thing, since it's got the protection of the bigger steel building around it. Later, when you want to make it shop-only space, you can still do that, just frame with your future plan in mind when you do it. If you frame the ceilings right, you can build stairs and use the "roof" of your "house" as a storage loft, play area for the kid(s) or whatever. Think roof deck- your options are open since you don't have to put shingles on it.
-Put a regular furnace in , but don't share it (vents or returns) with the garage area. This will mean you leave the french doors shut most of the time, but it'll also mean your wife and kid are more comfortable and that your energy bills are lower.
Now, still with me? I just added a few grand to the price tag of your project, and maybe it's not 110% what you had in mind at first. That said, I'm just thinking ahead. With the above changes, it'll be easier to sell later, easier to live in, easier (cheaper) to heat and cool, nicer, and better in just about every possible way. Forget about codes and nanny states, it's not about that. It's about protecting your kids and making a nice home for your wife/girlfriend. If she's willing to essentially camp out with you for 12 years while you do things your way, she's a keeper- make her a nice place.