You're right that umbraco is not going to replace your insurance application. But umbraco can certainly integrate with it to display information to website visitors, and pass information back to your backoffice applications. .NET controls would be the way to do that. Those controls can be quite simple or very complex, depending on your needs.
An example to look at might be the Commerce4Umbraco project by Paul Sterling. You can read more about this project here:
http://forum.umbraco.org/yaf_postst4325_Commerce4Umbraco--Preview-Release-1.aspxhttp://motusconnect.com/commerce4umbraco.aspxThis is a classic example of using the strengths of each application to the fullest. The system that you propose is certainly possible, though it will take some time to develop the controls. If carefully designed, the on-going maintenance of your controls shouldn't be particularly difficult.
As you point out, you could keep the content and the other applications separate, but I don't see that that gains you an improved development/maintenance environment. Also, though .NET master pages are coming in the next release of umbraco, they are not available in v3.0.5. There are, of course, other ways to accomplish this, but since you mentioned master pages I thought it worth clarifying the situation.
Have fun with this project, and let us know when the site goes live! And feel free to ask questions along the way :)
cheers,
doug.
MVP 2007-2009 - Official Umbraco Trainer for North America -
Percipient Studios