Digital product delivery and payment system

We are looking to develop a system that with an already existing database (and also a webpage with oscommerce cart) the customers be able to download a software (something like steam or origin client but much more simplier) and with that software be able to interact and buy digital products and then these products be downloaded and automatically installed in their PC. We are looking for a custom build using c programming or adobe air both options are ok but if there exist another option its ok. Also a payment gateway must be attached, the payment gateway will be sepomo (using sms payment system) so the customer will sent a sms and then it will receive the code to confirm that the payment has been done. After that this client must have the download manager where it could pause - stop- resume the download and when its completed it must automatically install the software (just launch the installation [login to view URL] nothing more) We are looking for offers and propossals