Ouch! Your room is big enough but not tall enough. Ceiling height is hands down the most important factor in proper studio construction. But work with what you have for now.
Start with taking the biggest space you have and just working with it. Just get started in that ONE room then feel your needs out from there. Its impossible at your stage to really plan your layout because your needs will evolve as you progress.
I'll give you a heads up. I've spoken in tremendous depth with four of the best studio designers in the world. It doesn't work like this. They need to spend a lot of time at your location, because there's way more factors than measurements. It can not be done from a distance. Soft costs start around $20,000 just for planning and design. The best quotes I've gotten were ballpark $70-$80k for a small mix room, vocal booth and machine room. They also need to be onsite at the end of the project to tune the monitoring system to the room. On site billable hours run $1400-$1800 pr day plus air fare and lodging. I'll tell you this now so you're not caught off guard if you start talking to others yourself. But you really don't need this right now...its years down the road. But take your time and grow your business slow and steady. Wait until your income base is rock solid and stable before making that kind of investment. There are many decent tracks mixed in highly flawed rooms. Everyone wants a perfect room. But many get by without one.
...don't worry about excellent monitoring for now. Just work with the best you can afford. Work with the best monitoring that makes sense for your bottom line. Rome wasn't built in a day!
If you're planning on being in this house for a long time, can you raise your floor and increase the ceiling height for the mix area?
If what you're saying is that you have a $28k gear budget, the WORST thing you could do is go and buy gear right now. First, without knowing how to maximize every penny of that in accordance with your actual needs, the only thing you can do is guess, and hope you get it right. I'd take about $3000 and get started. Put the rest in a short term easily convertible investment account and try to grow the remaining $25,000. In the meantime, get yourself some Yamaha HS8's, a good computer, a reasonable interface, a couple mics, and copy of Pro Tools. That'll keep you busy for a couple years.
Are you renting? Buy sound treatment that can move with you to a new facility if you leave.
You can't learn shit off headphones. BUY MONITORS!!! Yamaha HS8's, JBL 4000 series, or anything Focal.
No, you can't calibrate a home stereo speaker that was not designed to do what a monitor does. Its completely incapable of ever being a monitor, because they're made of completely different materials from the inside out.
Look, you're putting your cart infront of your horse. You may want a stellar room, but you have to get there one step at a time. You don't need a professional room. You need the best one your current business income is going to allow you to afford. I see what you're thinking by killing 2 birds with one stone. One bird, the studio (which you will be using to record later) and a training environment (which you can use right now). That makes sense. The problem is that the price of the room is justified by the workload you have NOW...not the work load you hope to have later.
You're a construction guy...right? Sounds to me that you're literally wanting to go buy steam rollers, cement trucks, and road hogs that you CAN NOT deploy on a current contract NOW, based on the grounds that you can learn how to drive them now, and practice laying down concrete and asphalt. Don't buy those vehicles until you have a work order secured! Yes, if your business is successful later, you will indeed be using them later. But the expense is not justified here and now.
Now this is dead on the money. Your own work makes excellent portfolio material. And that is exactly how I would recommend getting your portfolio moving forward.
People don't get rich doing things they hate. They make money...sure...but what billionaire dislikes his work? Show me a single one?
I'm not trying to discourage you from attempting this. What I'm saying is the choices you make now, and the wisdom you apply to your career make the difference of weather you're on the fast track or you do this the hard way. I sincerely applaud your ambition here. And your willingness to seek advice. Just don't get ahead of yourself