Scouts are the easiest to find and there are usually a few good, unemployed (which saves fees) one available. I typically search for staff with that role and look for people with the highest judging player ability / potential that I can get. Playing as a top team I rarely employ anyone with less than 15 for either. Getting scouts with different knowledge is useful - if you'e already got someone who knows Spain then try and find someone from a different country. Their knowledge of a country will increase as they scout though, so even someone who doesn't know a country will get to eventually.
For a DoF I look for JPA and JPP again, with a high determination. As he won't be directly scouting I don't think his scouting knowledge will increase apart from being in the country of your team.
Assistant Manager, for me, is more focussed on the ongoing player management (determination is critical for all staff, look for man management, discipline, motivating, JPA, JPP and tactical knowledge - for the match day assistance). Also remember to check personality and playing preferences - you want someone who will work well with you.
Finally, for scouting knowledge, don't forget about feeder clubs (if you're big enough to have one). You automatically get the knowledge from any feeder clubs so having those spread around the world can be an easy way to increase the overally knowledge - especially if your feeder club is at an intermediate level and has feeder clubs of its own.