You don't have to dictate yourself how or what to build. I suggested that you know what you at least want to achieve (aka one certain goal), at the start of the build. As it is your first ship, the goal of building any survival capable ship is ok man. My first bigger one was a fighter of only 40 m length though, and I had no clue what I wanted...I had a blurry image in my head how the hull should look, and nothing else as goal. I think the goals of a ship rise up with each ship you build, settling low for the first one is totally fine.
Also, as certain design goal, the military role or the later usage was not what I was trying to point out. A certain design goal can even be, that you have a certain thruster shape, or want to build a robot that shoots out lasers from his feet. One thing you want to achieve with the ship, that one thing that keeps you going. And if it is the sole purpose of a survival ready ship it's enough as well - later you might have bigger or more precise goals in mind. It is not helping, if you now pressure yourself to find some certain goals. If you allready and actually want something certain, you know that you don't want to miss it, and will stick to it. But if you have this one goal, take an inner note, and don't add too much other goals around it or you might end up with a ship that looks very cluttered and doesn't feel like a whole. The other goals are optional. This way you might often end up with projects you feel satisfied about, because you get what you came for in the start.
For my first bigger fighter, it was a certain hull shape. For my first corvette, it was the railgun from the Donnanger. For my first big salvager, I saw a station, that had a cool shape, and I copied it, and I wanted to test out an RP-building style, where each system is accessible from walkways and seperated by walls. My first small salvager had the goal, to being affordable with the starting money, and having open spaces where I can put in upgrade systems (jump drives, more thrust and a slave to the salvage array).
Here is a video with the content I just talked about. I found it helpful, but I didn't spoil my first building experience with too much theory. I am happy to learn from myself and make up my own mind first, and then seek out for other opinions to see how others do it. I watched it after my 3rd bigger ship. Maybe you want to look into it a little bit later:
