Happened to me, first time so i had no idea how to quantify Kullervo's strenght too. I only realized when i saw Undercroft enemies being lv140+ lol

Personally, my default choices for every warframe that doesn't have particular needs are Guardian and Energize - although if i didn't have access to Energize, i'd probably go for Molt Augmented. Arcane Reaper is also a great substitute for Guardian, as it can be farmed in Duviri. Magus Repair is also very useful, and i've personally come to rely on it a lot.

That said, prioritize filling your mod slots and maxing your mods. They have a much larger influence than arcanes when facturing time spent to acquire them.

You can sell the leftover parts and buy a receiver, you'll get a little platinum too

Have you tried comparing single shot damage against both armored and non-armored? I find it very hard to believe that with a powerful riven like that damage would not increase, even against heavily armored targets

I'd say keep it for when you really need it, and you will need some at some point. As others pointed out, inventory slots are the biggest plat consumer, and as you expand your arsenal, you will feel it. Then is catalysts and reactors, which normally aren't available in large quantities - i'd say buy catalysts but wait and be patient with reactors, as they have a much lower consumption rate and you'll eventually get all that you need for free.