Are there any metal building suppliers near you?
Most are customer friendly, will work with you on design.
They have worked out the most economical system of construction, yet provide a durable product.
There are Youtube videos of metal building construction. If you familiarize yourself with the concepts, look at available components online, then go to the showroom, ask questions, get quotes.
They can provide weld up, or bolt together framework. Sounds like you would want bolt together for ease of access.