I'll pick off a couple of answers from the group, perhaps others can take the remainders:
>Are there advantages in terms of cost, with regards to hosting?
Unlikely : DotNetNuke requires the same platform as your existing asp.net application. The only saving might be consolidation of databases, if you're paying per-database. It's unlikely you can get better hosting for cheaper than that. Quality hosting costs money, regardless of technology.
> Would our .cs and .aspx files all be transferable "as is", without making any modifications to the code?
No : if you want to integrate your code properly into the DNN framework, then you'd need to create any custom modules. Chances are, however, that there is a lot of overlap between your proprietary system and DotNetNuke. You may find it's as simple as copy/pasting a lot of html from your existing site into the DNN installation. Without specific details, it's impossible to say. I would be looking at what sort of content you need to display, then chewcking out the core and third party modules to see which is best suited to displaying it.
> Are there good engineering companies out there, that can work in DotNetNuke
Yes, but it's unlikely you'll get anyone recommending a company unless it's their own. The best thing to do is to check out different people and ask them for quotes. It's not hard to find DNN system integrators / developers.
I think in summary it's best to look at the end goal and work backwards from there: a DNN site with all your content, how best to achieve that? Rather than trying to work from your existing stie forwards. Your objective should be to eliminate as much custom code as possible, because custom code is like custom furniture : very nice, but very costly.