Don't let price be the governing factor although there are plenty of good, inexpensive engines out there. I would suggest a simple single engine test bed of a plane to be sure the engines are reliable. Perhaps the most important consideration is the location of the fuel tanks. the top of the tank should be at, or just slightly above the C/L of the needle valve. If it is too high the engine will flood when it is full and starve when it is nearing empty. If it is too low it will not draw fuel properly when taking off and starve when it is nearing empty. Another hint, use OS A3 glow plugs. They are hotter than normal plugs and are more reliable for use on a twin. don't try to peak the engines out, settle on reliability. Also it is not necessary to have each engine exactly the same RPM wise. 2-300 RPM difference is OK. You can compensate for any variation with the rudder.
Peter