As a consultant who has been involved with the Utility Network model since it's early alpha and beta releases I'll leave my 2 cents. The hardest part of the implementation of water/storm/sewer is going to be working through all the data preparation / cleanup, and for that I'd highly recommend using Esri or a Consultant to help manage that part. If you have a strong data conversion team in house they should be able to handle the conversion, with some guidance from Esri or a Consultant. If you don't have a strong data conversion team in house you will likely need to bring in an outside consultant. If you do end up going the consultant route I recommend you look for consultants who have been Certified as capable of implementing the Utility Network by Esri. If you want to do it all in house then a good place to start would be to take all of the classes and certifications recommended by Esri to get certified.
