The engine, of course, will be the first issue. It must be within the manufacturers recommended TBO: not only the hours but also normally less than twelve years since new (which means since manufacture not when first put into service) or the last major overhaul if applicable. All AD's must have been completed and the associated paperwork in order, so no missing logbooks or releases to service.
Any mods done whilst under EASA should not cause any problems but any done prior to EASA could still be problematic. Comprehensive paperwork for the changes will be necessary.
Registration is straight forward but the C of A application requires careful attention, so start with a CAMO before changing over.