Paying cash is not something new or rare in the DR. As a matter of fact almost 90% of home sales are carried that way. Banks in the DR lend a measly amount of the total paid for any home, when they opt to loan you the money. At the rates they offer, most people buy cashiliiin' better.
Used car dealers are a dime a dozen, but if you really want to buy a non-lemon, aim to visit the long established dealers unlike the mini dealers all over the place.
If I may: Wait a bit longer to buy a car in the DR, at least 6 moths. By that time most dealers would have sold enough inventories, so to take advantage of the lower taxes on imports that should impact the used car market to bring prices even lower.
Diesel? Not a great idea... Diesel motors are too much of an expense to fit a gas system to (if possible at all).
They way gas and diesel is going, gas will be the only choice for the regular non-millionaire folk...