Vehicle door minimum should be 14'. Raising a roof is a true PITA!!!!!!!!!!!!!!!!!!!!!!!! Insulation and drywall (sheet rock)
also need to be in quote. Also consider electrical, at least a 200 amp panel with a 50 amp RV plug. Plumbing, heat, air distribution, floor sump, lighting, vehicle exhaust exhausted outside. The 4" floor doesn't sound adequate to me either. And then there is the question of the impact of such a structure on resale value of the property. Do you get your money back, make a $ or two or (as with a swimming pool in our climate) get the green weenie! It may make some sense to talk to several local licensed Realtors as regards their opinion regarding the impact of the structure on the property value. And before you do anything, check out the zoning/building code with the appropriate municipal authority to insure that the project is feasible and what bureaucratic hoops you have to jump through to obtain a building permit. The man door is not a common size! Was this intentional? Here the standard/common size is 2'8" by 6'8".