VPN Client for Windows, Mac, iPhone & Android

Hello all, Before we start, we have a strict budget for this product of around $500, we could stretch a little bit more if it's a must. I know this is not huge but right now our app only needs to be basic. We require a VPN app that works on Windows, Mac, iPhone and Android, the interface does not need to be amazing, just needs the users login details, select the server and connect. We would like the following additional settings where possible: 1) Start application at login 2) Auto-connect if the VPN drops 3) VPN Kill Switch - Disable internet if the VPN drops 4) The app should remember the clients credentials if they wish We are looking for someone who can also help improve the app in the future, so there will be further work. However now we just need the app to get us up and running. If you can help, please get in touch.