Qualify the contractor and highlight the benefits of working together which would include consistent work for them and regular paycheck.Personally, I'd buy the materials and never pay any money upfront, a "red flag" on its own.

I think most hosts give 5 stars unless there is a serious issue so even a 4 star is a red flag for me that I would check into. 

Just make sure you understand laws, zoning and you are eligible for lic.  

It would be a big red flag to me if they are doing a lot of volume in this market. https://www.biggerpockets.com/member-blogs/10145/83067-limited-partner-s-guide-to-investing-in-the-right-deal

I can build a mini storage or whatever as long as the zoning is compliant.

Someone in zoning should be able to help. 

Get out of your comfort zone.

You could easily buy land that is zoned for multi-family and find a lot where you could build 4-8 townhomes.

Red flags all the way around. 

We would do a Self-Storage location with containers subject to zoning.