3D Walktrough program

We are a design studio, offering our clients 3D Office Rendering. We've recently launched a Software/Service where they can modify their renderings and adjust it, using HDR technology. We want to take this to another level, offering our clients the ability to navigate around their virtual office, select an object and change it's mapping and color, adjust lighting and change the carpet. Once that is done, they can place their scene in a preview window and save the image in high resolution and even be able to print. Our HDRender program was done in C++ with Nokia QT compiler. This time around, we don't know what would be the best technology to use. We thought about UDK, UT Engine (Unreal Tournament) since it is perfect, in a way, all we have to do is build the world and import the 3D Scene from 3DSMax. Once that is done, can we build an stand alone Installer for our clients to install and use the program? Once they do install it, how can we go about sending them another scene the next time on another project? We need to come up with a work flow. Obviously we haven't figured out everything yet and we are open to suggestions. If you have developed some kind of a program resemble to what we'd like to accomplish or simply you find the project challenging, contact us and we'd be happy to brainstorm. As for the budget, we have no idea how much it will cost. That said, it doesn't mean that we have unlimited resources either. We'd appreciate your estimate and I'm sure we can agree on a number which will make us both happy. As for deadline, we'd like to be able to provide a demo to our clients by spring. Because we are in Canada, we'd appreciate Freelancers from North America responding. Thanks,