I did not read everyone's suggestions here so I would like to apologize if i missed something...
OP, here's my mod advice. Being picky about mods is an end-gamer's concern and nothing you'll want to be concerned with right now. With that said, having a plan for modding IS important. If you can farm 5* health mods, then keep farming them unless you can farm other 5* mod types.
Right now, I wouldn't even think about upgrading a mod without speed secondary, but I have over 300 mods with speed seconds so I should be picky... https://swgoh.gg/u/speedokillz/mods/?stat2_5=1
Here is the order of preference and my advice. #1 Priority is 5* mods only. Health mods are usually the first a player can get to 5* so don't wast any time farming lower ones because the stat bonuses on the 5* are going to be a better investment that a 4* of another type. The second priority is the Primary stat: Potency Crosses, Crit Damage/Chance Triangle, Speed Arrows, and Protection Circles/Crosses are what you want to look for as premium primary stats.
Finally.. speed secondaries. If you wait around for a Crit Damage Triangle or a Tenacity Cross that has a speed secondary... grab a snickers, buddy cuz it could be a while. When you run into that premium mod, you can replace it on your arena team and move the relatively crappy mod to a pilot or something.
Oh... and I always level a mod to 15. Some will say to only go to 12 to see what you're working with and they may be right. To me, I've made enough of an investment already that I'll put the mod on someone eventually...
Good luck and hope that was helpful.