Hi Vidya,
There is not an easy answer, you should use your experience in this.
When you have the requirements, you need to know quickly if you would need an easy/medium/complex message mapping, if you need a XSL o java mapping the difficulty increase. Also the adapter chosen has importance. For example, if you need a jdbc adapter and you need to install the drivers.
Summing up, you can consider for you complexity estiamtion:
1. Mapping difficulty.
2. Adapter configuration. RFC, HTTP, SOAP, FILE without FCC, etc are easy; IDOC, FILE with FCC medium, JDBC and JMS difficult. (At least form me 
)
3. You need a ccBPM o SAP NW BPM.
Regards.