Hi. Yes it is good to have an assistant for a couple of reasons: In the Team Settings under "Manager" you can set your Assistant to organise friendlies in the off-season, to handle those friendlies (so you can focus on other things), if you do not like handling the media, you can set your assistant to handle media talks, and you can set him to handle team talks + he can handle contract renewals if you want to, although I never let my assistant handle this. Money matters is my responsibility.
What do I look for in an Assistant? Tactical Knowledge, Motivation, Determination of at least 15 points. Adabtability and Judging Player Ability and Potential also helps (also 15 points if you can). You will see that almost all the good Assistants that have high ratings on these points, will have good ratings on the coaching side as well. Start at 15 points and work your way down until you find a suitable candidate. Why these points? When you select a team before a game, you can ask your Assistant to pick the team for you. The better his attributes, the better his selection will be. On these points the chances are very good that you'll find a very good youth coach that you can appoint as an Assistant.
These are my preferences, and others will differ from me. But that is fine, as this has worked for me pretty well so far. That is why I think Pat Rice is the best Assistant in the game, abd not just because I support and play with Arsenal