Depending on the results of the pre-purchase survey you may need to factor in some budget for initial maintenance activity (either things that will need doing early on or e.g. before the ARC renewal / as part of the first annual).
Even if nothing comes up I'd suggest allowing for the first few bits of maintenance to potentially be a little more expensive than 'average' (if there is such a thing with aircraft!) as your engineers may find things they consider need dealing with that the previous engineers didn't if that makes sense.