Questions on material ledger

Dear All,

i want to activate material ledger in a company code, but i have some concerns that maybe some of you could give me the best suggestion.

1. is it possible to activate Material Ledger in the middle of SAP operation (i mean in the previous years, we didn't activate the Mat Ledger, just by now we have the idea to activate it)

2. what are the impacts by activating Material ledger, and some concerns that maybe exist in activating it?

3. is there any recommended link that gives the steps for activating it? i got some resources, but apparently the steps don't apply to my system, especially the step when I want to set the valuation area as productive (FYI, my SAP version is 4.7)

Many many thanks!

Nasikhin