Just get generator and frost shards. With missions, credits are extremely easy to come by.
Tranquility is the best until you start getting into the 30s and sometimes its better to have gen against support mercenaries, but tranquility is nice at low levels. Generator is nice if you can't get tranquility, I prefer generator over piston punch especially against Strength builds generator is the better option.
Generator points are 50+2*the armor def/res.
So with 100 def and 20 res the HP is 50 + 2*100 == 250
and the EP is 50+2*20 == 90
Gen doesn't heal poison.
Field medic heals poison, assault but helps with poison, but assault bot is garbage so don't buy that.
Also a good passive core is critical heal which can be found at the solar world factory which is at elon musk in wasteland. I'd go gen first and frost - since somehow you managed to be level 20 without getting cores...