Building vs buying duplex in north Florida

[Jonathan R Boswell]

I was considering making a duplex my first house purchase.  Due to a family issue with my parents and sick sister I may want to go the route of having a duplex built rather than finding one already existing.  I need vacancy on both units for the easiest move in.  My parents and sister in 1 unit.  My brother and I on the other unit.  Eventually my parents or brother could move out and find another place.  But i want to keep the duplex as a first time investment in what will be a portfolio of rental properties.  I'm concerned about getting the numbers correct and not overspending on this first house so that i will be able to move out to my next property and keep the first one as an investment.  

Does anyone has any experience with building instead of buying a duplex that they would like to share?   Was it worth the effort / profitability?