If you need your game to be as open-source and budget-friendly as possible, I would advise developing it in Actionscript 3.0 with the Starling framework. You should be able to use AIR for cross-platform support. This route will allow for nearly-free development without any licensing fees. The access to the full source code…
One of many options: Flash: Actionscript3: Starling Framework http://gamua.com/starling/ Starling is a hardware accelerated 2D framework to build games with, you get easily full 60 fps with under 10.000 triangles on screen. A few month's ago I worked on a game "Gangster Squad: Though Justice"…
Honestly, flash + photoshop is a solid combination. I don't really see the point of using Unity or UDK for a 2D game. Perhaps Construct 2 is nicer for you guys, but I won't bet my life on it. I've given the program a quick spin this week and it comes with a bunch of nice built in features such as standard platforming…
Thank you all for the great advice. I'm seriously thinking about using the Unity engine. What troubles me though is the fact that it is not entirely meant for the design of 2D games. There should be no trouble in getting around to set the camera angle to suit our purposes, but I'm worried that it will be more effort to…