The old axiom "Failing to plan is planning to fail" comes to mind.

I wouldn't say sell yourself short but always be realistic. For example if its a deadline you want to give for a project will take 2 weeks, build in some space for running over (a % based on complicity and client) and give that.

In your case I think your talking features. You can promise the basics first and discuss a 'stage 2' which adds more functions later.. If you get those done before planned thats a big plus.

Most clients only remember failures, not success. So I would agree with you on this one.