I wouldn't pay them anything upfront...I recommend you always pay for labor AFTER the work is done.

Hold it for few years and then sell it or built a second/vacation home. which area would you recommend and why?

Determine the strengths of your property manager, if they are experienced in working with section 8 department and tenants with vouchers then I recommend choosing section 8 tenants otherwise look for cash paying tenants.

Any books, blog, etc where you would recommend starting to learn about these business structure etc, that are not written with the only purpose to sell you something ?

It will still probably be a terrific investment. 

I have an agent that's recommending a new construction in that zip code.

If you have other channels to list on, I recommend starting there and then launching on Airbnb/VRBO until you are within the booking window for your market.

I am based in OC if anyone has recommendations on advisers Real estate, financial, meet ups, anything really.

You can only play games for so long.Chances are your next door neighbor is an IRS agent and turns you in when your tenants blocks their drive or some other random event disturbing your IRS neighbor.ADU catches fire and your insurance won't cover you, because it was an illegal and unpermitted structure.The city inspector told the last guy not to build or rent it, and is now pissed and wants to make an example of you and just red tags the whole property and no one can occupy for the next 1-2-3 years while you wait for a hearing.Then when you get the hearing, "staff" recommends you tear the whole structure down, and if you still want an ADU, you can apply for a permit and rebuild it.Your tenant quits paying rent and won't move out and you can't evict them, because there is no valid lease and they're just your "roommate".Probably a dozen other bad things can happen to you when you do stuff like this. 

Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?