Probably not what you want to hear (especially having created a finished model) but had you considered making it hollow?
It should be relatively painless to cast the exterior facade, and produce a hollow shell - it would substantially bring down the weight, and make it more attractive to overseas buyers.
Additionally, with a lift off roof, you could make future designs with a different roof style (but same dimensions) giving potential buyers multiple options, and increasing the chance of multiple orders, as even with just one extra house design,you would be able to build a whole street of different houses - looks like the garage is separate, and can be switched over to the other side for more variation as well?
Im not suggesting bags of internal detail, or a second floor - these can be added by anyone that wanted to go that route, but I honestly believe you'd get more interest with hollow buildings.