Dear Sruthi
I believe there are mainly these factors
1.) business driven
2.) IT driven
What does that mean?
To start with EHS you need some basic customizing to be done. Therefore you need to collect requirements form business to do so; e.g. you need to decide how many substance auhtorization groups are needed; which spec types should be used, which process should be supported (e.g. MSDS distribution, DG checks etc) etc. The same is ture e.g regarding phrase core customizing. Either you buy phrases (as part of conent) or the business need a maintenance organisation etc.
IT driven:
As mentioned by yourself: based on the decision of business you need somebody who knows to implement the demand. Most critical ist "core customizing" (like substance authorization groups, "MSDS distrbution", DMS set up etc.
The next step is e.g.: is there the need to have expert rules in place and if so which one;
There is a "Best practise regarding chemcials" available which can guide you in a project
Therefore: if you take a look to SAP help of EHS: from IT perspective you need know how in SAP MM, SD, PP etc. based on the modules you (that means business) would like to use
Keep in mind that some very important strategic decisions need ot be done. Eg. which release you would like to use, is there the need to start with SAP EHS COmponent ExtensioN; Is there the need to use e.g. SAP Reach and COmpliance Suite
Combining that: before you start tzhe whiole thing you need the scope of the project; in most cases a wave based approach is used either based on modules (start with SAP EHS, go on with SAP EHS SAF etc.) in combinaiotn with region (stArt with NAFTA then europe etc.)
Check e.g. http://scn.sap.com/docs/DOC-41109
You will find some link to "similar" demands as yours.
C.B.