About the sonarman. If you don't really intend to use one later on a CL for example, you don't really need one just for your DD. Indeed it will increase detection range but not the low level one you will likely be using. So an at-level sonar will really be no better than the default detection you get for being a DD.
Remodel - you don't need to because you don't get the remodel credits back. Usually at lower levels you zip through the ship tree fast enough so you don't end up playing one ship for a prolonged period of time. For a DD remodeling is probably something you do later when you decide you want to play that ship for fun due to a certain characteristic.
ASW is very beneficial even if you can't attack the sub and just detect them. Also consider AA. Good XP and credit gain, beneficial, fairly low risk, and great for learning manual aim.
If you're lucky some guy named Bonaventure might still be out there giving out credits :)