Salesforce.com Resources

My company is considering a switch to Salesforce.com. We've used Marketo for 3 years so I'm familiar with Marketo. What I'm not familiar with is what I need to consider if and when we make the move to Salesforce.com. I've come across a few resources, but thought I'd ask the community...

1. What planning needs to be completed before implementing Salesforce and Marketo?
2. How long does said planning typically take? I'm sure it varies, but what's the average?
3. What other resources and considerations would you suggest?

Any advice would be much appreciated!