Migration from single forest to office 365

Hi,

I need to migrate half of my users to office 365

 - No of users 2000 (Out of 5000)

 - Single forest , Single Domain environment

 - Exchange 2007 setup onpremises (Single SMTP domain)

Client need coexistence for atlas 2-3 years , What is the best method to migrate..?

Also have plans to remove current edge server completely from on premises and use EOP for mail routing. Is that recommended...?or do we require at least one edge in on premises as SMTP gateway.

Thanks,

Shafeer