I just graduated with an Aeronautical Science degree and I am planning on starting a freight forwarding company. It would need to be high payload capable and relatively fuel efficient, the route that would be flown everyday is only 300 miles one way.
You would have to provide more answers to the comment I made above.
But for a relatively small payload (<4000lbs), I'd say you can't go wrong with a Cessna C208 Caravan. You can buy a good used C208B Super CargoMaster for just shy of $1 million.
Another recommendation for that route would be the DeHavilland DHC-6 Twin Otter. Payload is increased by 1000 lbs or so over the Caravan and you have the additional safety of two engines for over water operations. The downside is that these aircraft are fairly prized and will set you back about $ 3-4 million.
Another heavy lifter with an excellent reputation behind it, even after 80 years of service, is the venerable Douglas DC-3/C-47. You call, we haul 6,000lbs to just about anywhere within fuel range.
All of these aircraft are extremely rugged and have a good reputation after decades of hard use in bush flying. They are capable of operating on short, rough airstrips - a good benefit in case you decide to expand to more remote destinations within Cuba and can carry tremendous loads. They're also turbine powered using PT6 engines (except for the DC-3) so fuel is readily available anywhere in the world as is maintenance and spare parts.
Are you a pilot? Are you familiar with aircraft regulations and operations?
I would avoid radial engines. I would seriously consider planes powered by TPE-331over the PT-6.
The Metroliner will haul about 5000lbs over that distance. They're airliners and have maintenance bills to match.
The MU2 is also a popular freighter. Somewhat smaller than the Metroliner it is still an efficient plane. It requires special training.
The Beech 1900 is also a popular freighter but I would prefer a Metroliner due to the longer engine life and better fuel economy. Like the Metro, they're airliners and will require some good maintenance.
The Beech 99 is a popular freighter. I've never flown one but it has a reputation for reliability.
For piston aircraft, the Cessna 404 will lift about 2000lbs. I love the plane but the engines can give you trouble (@ $50,000 each) if flown by a low time or poorly trained pilot.
The Piper Navajo (350hp) is also a common freighter. It won't lift quite as much as a 404 but it's a solid machine.
The Cessna 401/402 line is also good. On par with the Navajo.
You'll also need an air carrier certificate. Have you started that process?