I don't have the expertise to truly address your application but here's a few comments.
First, if your application is DC then you obviously don't need an inverter. You probably just need an appropriate charge controller and transfer switch and such. Probably a lot less money for equipment than entry-level for Sonnen, but I could be wrong and you'll probably have to engineer it in house.
Sonnen is designed for AC applications. Their individual battery units have BMS and communications and I'm sure could support DC applications somehow, but I don't think they will be inclined to support those in an application other than their home storage units. There is another company, Simpliphi, that makes very similar LiFePo battery units that seem to be truly intended to replace older lead acid/AGM batteries in configurations that were designed for such. That might be a better bet for you since applications for such are very broad. But unlike Sonnen I don't have direct experience with them.
Whether your application really calls for lithium is somewhat out of my range to answer.
