Hi Lokesh,
You yourself replied whole scenario. You have either REQ Number or Date range. Based on that you can retrieve details. Further it is possible that you may not get selection from FM based on date range then you can check CRMD_ORDERADM_H table and retrieve data from that table. Once you have RFQ numbers rest logic is same as you mentioned in your reply.
Further once you have all details you, ALV summation can be done easily.
You may need to run some FMs in the loop so don't worry about that. Also you can check FMs like BBP*GETLIST to have complete list as well.
Do let me know if you need any help specifically.
Thanks
Siddarth