First with the buying of land; get a good lawyer to do all the due diligence of checking out the land, title, survey, liens, deslinde, taxes due, etc. Be warned that there are tons of scams being perpetrated by what appear to be honest sellers. A good lawyer will save you from any future headaches.
After you buy the land, and are ready to build, get an architect/engineer to draw up the plans, if these are necessary for the type of home that you want to build. Simple type homes do not require this. For the actual construction, hire a "maestro" to manage and supervise the construction, instead of the architect. The maestro will also assist you in hiring the different tradesmen needed, and in negotiating prices for materials and labor.
The best way to find good, qualified people to hire (lawyer, architect, maestro, tradesmen, etc.) is from referrals. Keep in mind that ALL PRICES in DR are negotiable, so don't be afraid to make lowball offers and then negotiate from there.
I also suggest that you read this entire thread
http://www.dr1.com/forums/living/63016-homebuilding-dr-inexpensive-dominican-way.html It's a sticky for a reason.
Good luck!