A good rehabber will know what to do and not do for improving the property, will be able to do it more efficiently and save time and holding costs, and probably will save you money on the remodel itself.

learned to build homes more efficiently with coordination of contractors and materials preparations.

Know you have an upcoming capital expense and ensure you have some capital in an easy-to-access place.

(We are GC's, so estimating repairs (and managing contractors) will not be hard)5. (1) Efficiency | (1) 2 bed, 2 bath | (1) 2 bed, 1 bath | (2) 1 bed, 1 bath 6.

So therefore we're plannig to go with a single family home, but thinking to turn the garage into an efficiency and rent it out, I don't know if this is illegal, or is there any other recommendation?

Fix your lease to ensure it stipulates stiff penalties or termination for violations. 

I also want to ensure I can scale and don't want to hamstring myself by making a poor decision now and deploying the capital on a bad deal so there is a bit of trigger shyness.

Typically you want to hedge the risk of tenants not paying during the application process by ensuring consistent income for at least a year (if not longer).

Perhaps once you go through an agency you can make some notes on the process to ensure you are well prepared and can do it yourself if it ever happens again.

I'd much rather keep a good relationship with a good tenant and try to make any repairs/adjustments to ensure this type of thing doesn't happen again.