Infected clip (toxin) and hellfire (heat) combine to make Gas. Gas is a very finicky element, you might want to consider toxin and electricity for Corrosive, which is very strong against armor, the first thing that will probably slow you down. Or just Toxin if you are fighting corpus, because it bypasses their shields.

I wouldn't worry about following builds this early unless you start having problems. Build weapons like the Hek and Atomos, they're great fun. Level up your main damage mods (serration/hornet strike/point blank/pressure point)

Multishot is important, getting it would be good (There's one for rifles, shotguns, and pistols) https://warframe.fandom.com/wiki/Split_Chamber - it doesn't look like it drops from spy missions though.

So, first, you perhaps have ONE weapon that is worth keeping long term or investing in, that is “following a build” for:

Dread

That’s it. That’s the only weapon I’ve still got that I had at MR5. There are a few others, especially now that there’s the end game option to revitalize some early weapons like Dual Toxocyst, Braton, etc, but really, just Dread.

When it comes to forma bps, I can’t recommend enough that you NOT level up in the “efficient” spots of Helene/Hydron unless they are fissures. (Which they are, relatively often.)

Instead, level up in Lith and Meso and even Neo Defense fissures. Why? After 5 waves, 25% affinity bonus. After every 5 waves, you get a choice of a prime part or forma blueprint. You earn void traces. At wave 20, you get the “C rotation” reward, which is regularly a rare mod like Continuity, Streamline, Intensify, or Hell’s Chamber. I bet you don’t have all those yet. And then there are the 25% bonuses for everything else that come with every other 5 rounds.

If you make a habit of leveling up in fissures, you’ll never be out of ducats when Baro comes by—sometimes with some of those powerful mods you don’t have yet. And you won’t be bored out of your mind by looking at those same four walls of Hydron/Helene again!

That's the beauty of Warframe, you can start anything anytime you want, sometimes it just takes some time to understand what the game wants you to do and then the time to grind.

Feel free to experiment as well, if the grinding gets too annoying, you can always play some other missions for fun to see what you encounter, and maybe another cool build idea will pop up.

At MR 5 it doesn't matter much, but also it doesn't hurt to go looking at the general build and using the closest mods you have

At MR 18, usually when I still get a new frame or weapon, I play it for a few hours before going to Overframe and building it accordingly. The difference after with just changing a few key mods can be massive.

You don't really have to sweat min maxing everything unless you're having trouble with something. Usually the answer is just to use a different kind of frame/weapons.

On the one hand, the most "important" thing for you to do right now is to just keep on proceeding through the starchart. As long as you aren't struggling with enemies, you're good to just keep on going with that without taking too much time to farm unless you want to.

That said, it never hurts to get things early. Multishot really is a very powerful stat, and so long as you have enough Endo and Credits to rank it up it will be a big boost to your overall damage output. Spy missions can also get you pieces for the Ivara warframe, who is useful for a lot of stealth-oriented situations.

HOWEVER, please note that the rifle multishot mod doesn't drop from spy missions - the shotgun multishot mod does. It looks like the easiest (albeit not the fastest) way to get Split Chamber from the planets you've currently unlocked is to do invasions and side with the Corpus enough times that the Grineer send an assassin killsquad after you, then wait for them to show up and kill them when they attack you. Just be careful - if they win, you'll take a long-term debuff unless you spend some rare materials to remove it. Details on the assassins here.

It's your choice like the mod will be handy for the whole game and many builds. So I yeah why not

Always "build" with mods you have. Just dont stress about it.

In early game its good idea try to farm different missions to get all basic mod you can and go with your own pace.

I was going to say that grinding Forma is always worth it however Convict45 has the better idea. Farm Forma incidentally while ranking stuff. When it comes to Potatoes I get them via Nightwave. Do the daily/weekly tasks and you periodically get currency which can be used to buy things from Nora's store. Both versions of Potato are always options... however there's a short intermission between Nightwaves so that's worth watching out for, don't want to need a Potato and not be able to grab one because Nightwave ended a few days ago.

I'll say this: In spite of your ability to put Potatoes on an unranked weapon don't Potato a gun until it's 30 unless you're certain it's a keeper either for shear power or fun-factor. Potatoes don't just grow in the ground.

Join a clan if you have not already, they usually have researched a bunch of good weapon/frame blueprints you can buy and build if you want some diversity

After the star chart is when builds start becoming useful. You can just brute force it with whatever mods you have before then.

the base mods are usually serration (rank 6 is good enough when you start), multishot and ele damage. viral (tox + cold) or corrosive (tox + elec)