How should the 貰っておく part be interpreted? What (I) can receive, I ... (?).
To capture the nuance of the original, I would go with something short like:
"I'll take anything that's free!"
You might think I did not translate expressly the 「もらっておく」 part, but if I did, it would sound pretty wordy.
"I'll take anything that's free as it might come in handy in the future."
The sentence means
If you can have it, just get it (even if you are not sure you will use it anytime soon. Let's just get it first and we will see what happen next).
the -ておく implies what is in the brackets. It's like predoing/preparing for something that may or not come.