1. No. Your planets already produces food. The food determines the limit. If you build farms, you increase the limit. Mind you, increasing the food is not always desirable as each billion more population hurts morale more than the last billion did. Having 25 billion or more citizens on a planet would make things nearly impossible to keep them happy. Not only that, but the tax revenue suffers diminishing returns so it is not really worth it. The calculations involves the square root of the current population, along with the current tax rate and a number of economic bonuses.
2. No. The base hull already has a speed and life support rating (and other stuff) included. Adding things like engines and life support only serves to make the ship better. However, some things need modules, like weapons and constructor modules, to provide additional capabilities.
3. Yes, you can do stuff to change the target priority. Normally, the game will target what ever ships would cause the opposing fleet to lose firepower the quickest. There is a calculation that will determine the best bet. You can change the order by increasing defenses and lowering firepower on the ships you want to reduce the priority, and do the reverse on ships you want to increase the priority. However, this game does not have a sophisticated ship combat simulator, so I would argue that this is a bad idea. Doing so would create inefficiency. I would argue that you should make your fleets out of your best and largest ships (to maximize the value of logistic points), get a good balance between firepower and defense on your ships (which ship increase total damage done in the long run), and get good logistics so you can have large fleets (which can be more dangerous than powerful ships). You should also try to deny the enemy from being able to form fleets as a large fleet can usually take down a small fleet with better ships (unless they are a lot better).
If you want an exploit, you could always install atlas modules or fleet warp bubbles to ships. Those multiply the threat rating of a ship well beyond what they should be, so it becomes easy shift the target priority.