Rabauss,
I suggest you go back to your man at Finch aircraft and ask him to clarify what he means by more expensive. Thielert/Centurion had some components (torsional damper, gearbox, oil nozzle for connecting rod little end) which were upgraded on their 1.7 engine, and due to the company operating in a 'Chapter 11' type bankruptcy state, these upgrades were not covered by warranties as the might otherwise have been. This has been discussed at length over the last couple of years on Pprune, so type in Thielert or Diesel in the search function and have a good read.
Vibration may be more at idle RPM, due to the engine's higher compression ratio resulting in more torque reversal. A lot would depend on the design of the engine mount and any anti-vibration mounts used.
I don't see why diesels should be significantly cheaper than a petrol engine to make, although a few parts are still sourced from the automotive industry which would bring prices down with mass production. As there are comparatively few around, the development costs are still being absorbed in the engines being sold, and the parts count is significantly higher with many high performance precision components (such as gearboxes, injection pumps & injectors).
Homebuilt aircraft using uncertificated car diesel engines are a totally different matter. In many cases the engine is secondhand with a homebuilt reduction drive and operating costs are 'peanuts'.
Gaz'aile 2 translation to English here:
DIYer Builds Peugeot AX 106 Diesel Engine Powered Wooden Aircraft - Auto Motto