SQL Express works fine too.
A lot depends on how far you are going with InfoPath and electronic forms. I have been very successful, without DBXL, but it was a ton of work and I was always running into the inherent limitations of SharePoint or Database Forms. And then the solutions were popular and they wanted more. DBXL gives you so many resources and capabilities right out of the box that I would not want to do another enterprise class project without it.
If you could, I would try to architect it on DBXL and SQL. It will save you time and effort on the testing as well as make additional solutions a lot easier. If you have a server ideally, or even a PC, that you can put SQL Express and IIS on then you have everything you need.
Again a lot depends on how your initial test go and the customer response. It sounds like you have a limited proof of concept and you have to sell that first.
Let me know if you need any assistance. We have done lots of solutions and may have something similar that could be useful for reference.