The C20 Turbine will need two FAA air directives complied with before October, which will then make it into a C20B. Save the expense and get one with the C20B already installed.
If you do go for the C18, I can confirm that there is NO new part support of this model 250 by Rolls Royce. Yes, you can pick up low cost modules, but these will no doubt be ex military, old and probably have been stored up for a long time.
There is a big difference buying a C18 powered aircraft you don't know to having owned and cherished your own C18 powered aircraft.