Experts: Get the best expert you can. He ( or she obviously ) must be highly expert in relation to the issues upon which he has been asked to give an opinion. He must fully and properly understand the role of an independent expert, study the rules about giving expert evidence and be both willing and able to comply with them.
Then it is necessary to consider whether the expert has experience of giving evidence in court or if not whether he looks likely to be somebody who will be able to deal with the court hearing process. Check also to make sure that he has no blemishes discoverable by an internet search or at least no blemishes sufficient to give him too uncomfortable a time in cross-examination.