File Preview/Purchase/Install App - also OSX version possibly

I have a concept for a relatively 'simple' app that would allow users to preview files at a lower quality from a sever. Tags from files might also be displayed to aid searching and give user info. Then once selected offer in-app purchase and finally have the files deployed to the Mac in the correct location to make them available in the appropriate host apps. The task is simpler on a Mac but would also be valid/possible on an iPad too. If you quote you can add any additional message about which you can offer. I am quite clear about what features are needed but I'd prefer to work with a developer who can also offer me a great GUI design too. I can give you more details about the file types and therefore how they need to be previewed/deployed, etc. upon request. I'm not really at all sure about what a project like this really should cost so I've given a wide margin. Also it might be just Mac or iPad you are offering too.