(disclaimer: I'm not a pilot nor a builder.....yet. That said, I've been doing a lot of research)
With regards to range and fuel efficiency, here are a few thoughts:
1) In general, lower speed = longer range at the expense of time. According to cozyaircraft.com, running at 40% power and 185MPH instead of 75% power and 220MPH will net you a whopping 30% increase in range (from 1,000 miles to 1,300), assuming no head or tail winds
2) In general, higher altitude = higher speed, at the expense of oxygen, cold temperatures, and reduced engine power (unless turbocharged)
3) Turbines are thirsty, thirsty engines. Fast, sure, but very, very thirsty. In a plane the size of a Cozy, you want piston power.
4) Aircraft diesel engines are promising, but also very, very expensive at this time. ac-aero has their Falcon FL200 that's supposedly 75KG and 210(!) HP, but its BFSC of >.39 will probably erase all the range advantages of a diesel. Continental has a whole lineup of diesel engines, but again, $$$$, and they're heavy.
5) Further on the diesel engines, the CD-300 weighs a whopping 550 lbs dry, and would push your empty weight to 1300lbs. That leaves you only 340kg for passengers, bags, and fuel. The stock fuel tanks hold 150kg. If you, your passenger, and your bags consume another 170kg, you'll end up with only 20kg of extra fuel before you hit Max Gross. Fast is nice, but you pay for it three times: first when you buy the engine, second when you can't carry as much fuel because of the engine weight, and third with the higher fuel consumption to go faster.