Definitely carry out as much Due Diligence as possible on the developers. Look at their track record, check the background of the company director (via companies house, Google, linkedin etc). Thoroughly read the contract and get it independently checked by a solicitor. Do your own checks on the development land - check with Land Registry who owns it, and check if there are any other charges on the land / property (you want to ensure you have First Charge on the property as this is your security).