100 K is a high budget for the casual games market. You are very fortunate to have such a budget. Game studio is a very stable 3d engine, but have you considered a 2d game? Most of the casual games are 2d because it's much easier to distribute. The game might be a 10 MB download vs 50+ MB for a 3D game. Also many people who buy these games aren't serious gamers with up to date PCs.

As of this moment, 3DGS is a good engine, but there are slick engines in beta right now. Torque, and TV3D come to mind. 3DGS is really beginning to show its age.

If you are looking for a free 2D engine, go here: http://developer.popcap.com/
They are also a publisher, so they can be a potential publisher for your game.