You can use one webdynpro application for your requirement.
1. create webdynpro application and convert it to fpm_ovp_component.
2. create work flow template with one activity task .
3. in SWFVISU configure this task with your webdynpro application.
4. initially for ess there is not work item id will be generated. after entering the data on save wrtie code for event triggering by using sap_wapi_event_create and link it to ur workflow template.
5. As u defined activity step, in MSS UWL u will get the screen and u need to write some code in wddoinit of component here u need read WI_ID based on that u can write ur validations.
6. on approve or reject u have to use sap_wapi_write_container for that workitem id and complete workitem using sap_wapi_*complete.
7. in work flow we can get if its approved or rejected then based on that u can send it to second level.