It is difficult, but not impossible to convert characters made in skill-based systems to D20. But that shouldn't be an issue if you're starting the game from scratch. If I were you, I'd focus on using the setting information from the Deryni game and choose whatever D20 variant that seems like the best fit (since it appears that you would prefer some kind of D20). The trick will be the magic system. Not having seen the Deryni magic system, I'm not sure which system would be the best fit.
The d20 thing's just so I can import, export and adjust more easily. I'm not a d20 purist or what-have-you, but I do like the system, and I do have a *lot* of sourcebooks that use some version or other of it.