With DNN you would be able to control what users can see and do based on their role. You can set up as many security roles as you need, and access them programatically in your custom modules. It sounds like you would probably want to build a custom module to handle application, but it sounds pretty simple.

Your application database can co-exist with the DNN database. Your tables can be in the same DNN database (easiest), or in an altogether seperate database. Your existing database could probably be transfered over, but you'll also have to build a web-front end for it.

To support DNN, your hardware requirements will depend on your estimated number of users/trafffc and what other applications you will run on it. The good news is that DNN is pretty scalable and you can run it on a web farm if you have a high traffic site. You have the option of hosting it yourself (you own the hardware), or going with the dozens of DNN/ASP.Net hosting services. Now with your requirement of having lots of pictures and files, it sound like you may want to host it yourself.

BOL

Ditto all the above. Have a look at the showcase site pages of this site to get some idea.

Any doubt of scalablity should by addresses by looking at the New York stock exchange site and have a look at each page available:

http://www.nysedata.com/

Obviously not in your industry sector, but it clearly shows that this is a serious application and not some nerdy hobbyist tool.