Starting with local credit unions is a good idea.2 - I recommend starting your search with a Realtor who specializes in working with investors.

I see you're in the Boston area, so if you're looking for cash flowing markets I'd recommend Lowell, Fitchburg, Leominster, just to name a few.

Which sites do you recommend for pricing? 

For flipping I would also normally not recommend adding an ADU.

I can definitely provide a recommendation.

Pick a screening platform/application that makes sense for you.As for meeting the applicants, I recommend meeting them at the property to show the place to them rather than getting coffee or food with them.

I recommend having a pricing software (pricelabs perhaps). 

I'm a little biased (am the owner) but seriously pumped to see you recommending it.

Florida is a unique state for MHP law and the insurance has gotten very expensive there too.I recommend checking out @Frank Rolfe's mobile home university content if you haven't already.

If you choose to move forward, I recommend having a backup plan in case Hawaii outright bans STR.