independent modules that could be looked at as sub-applications themselves
An service-oriented approach sounds like a good plan for the modules above. Implementing the modules as services and exposing them are a little different. Service-oriented analysis and design is a big topic and there are many different ways to implement services. I would suggest that you stay on track with what was planned, if possible. If you need help understanding service-oriented software concepts better, I would purchase a copy of ENTERPRISE SOA, Service-Oriented Architecture Best Practices by Krafzig, Banke, Slama.
Is it feasible to integrate BPMS and a MVC framework (Struts + Spring)? Moreover, the app requires rich presentations i.e., heavy usage of AJAX.
In a service-oriented architecture there are typically five layers. Not all applications are suited for a business process manangement and business rules approach. Heavy usage of AJAX indicates that this is a web, presentation-oriented application. You need to determine what the business rules are and how many of these business rules are related to GUI-related presentation. GUI applications typically don't fit in well with SOA designs. Maybe the service-oriented approach is incorrect. You have to figure this out and make your case.
Good luck!
[ July 29, 2008: Message edited by: James Clark ]