You should definitely start building as soon as possible. The longer you wait the more time others are out there thinking of and/or building a competing product. Every week there is a post on HN about someone who thought of something and while they were thinking or building someone else launched. Getting to market first might be the guarantee success, it sure as hell helps.
Also, talk to a lawyer. If you are taking $250k from friends you need to have contracts/agreements in place. Don't take money on a handshake. One or both of you will get burned at some point.
I don't know if these friends are seasoned investors or just people with money but if they aren't familiar with the startup world make sure they know what they are getting into. They could lose all their money by the end of the week and they need to be prepared for that (make sure your agreements are in place before you start spending in case this happens).
With $250, I'd say you got about 4-6 months (assuming 4-6 engineers) of runway. That means you need to have a good prototype in 2-3 months and be ready to start pitching real investors. So I go back to my first point, start building.
Also, talk to a lawyer. If you are taking $250k from friends you need to have contracts/agreements in place. Don't take money on a handshake. One or both of you will get burned at some point.
I don't know if these friends are seasoned investors or just people with money but if they aren't familiar with the startup world make sure they know what they are getting into. They could lose all their money by the end of the week and they need to be prepared for that (make sure your agreements are in place before you start spending in case this happens).
With $250, I'd say you got about 4-6 months (assuming 4-6 engineers) of runway. That means you need to have a good prototype in 2-3 months and be ready to start pitching real investors. So I go back to my first point, start building.