Seeking due diligence guidance for Investory opportunity with ROI of 60%+

[Shailesh L.]

I have been informed of investor opportunity to invest in New construction condo project.   He wants to raise $20M through investors and remaining $30M will be through bank financing . He is asking each investor to invest $500K and he is promising to give returns at least 60% ($300K) on investment once construction is complete and all condo's are sold. This project  will be completed over 3 years of time.  Developer is currently aquiring land and appraisal of the site is done.  As part of due diligence , what should I be asking to the developer, what are the potential pitfalls and how do I protect my investment ?