You can build virtually anything in DNN. It is essentially a security framework and it does not place restrictions on anything you want to fit into it. However, you will need to find an experienced and capable DNN developer to carry out the work and more importantly, you must also have an experienced person within your business responsible for managing that person's deliverables and performance. The committee you're on is probably for that purpose, but such committees are typically not staffed with technology specialists.
IT consultants tend to restrict their advice to the field and applications they know. Anything outside of their field is generally dismissed as unsuitable... and any developer or systems engineer can whip up some quick fud about anything at all in 2 seconds flat and have business managers sweating in fear as a result. I've seen that plenty of times. One basement dwelling dev in company I worked for held the entire company to ransom for years over his loonix ideals and his desire to build every application in the business all by himself. It was completely nuts.. they were still running Office 97 when I left and I understand nothing has changed since.
But I dye grass
Your chap's actual statement "Dotnetnuke is a framework you can use to make up modules that are pre-made, but you don't have the ability to do customization." is pure blather... just read it again. His second statement about the number of developers is much more accurate, but it is probably more a reflection on his firm than anything else... and besides, how many developers do you actually need... one?
You need to find a DNN specialist and have them assess the state of your existing application and advise whether it can be revised or improved in lieu of your future goals and requirements. That report would be put up against a proposal by other groups to replace the system with something else. One person or group cannot do both.
A lot of IT business are making a lot of money doing very little. I've seen real six figure projects that I could put together in a weekend and run from home. It's quite shocking. It would be interesting to know just how custom or generic this real estate application acually is.
Rob
P.s. I could be completely wrong about him, but what you've said strikes a chord with me and my experiences.