Someone's translation of that is:
"It is enough for today. I will let you go now."
That would be an okay TL: It is passable. At least, the essence of the original is maintained to a large extent.
The original does not explicitly say "I will let you go now.", but one could say it is implied.
I would recommend that you remember:
as a fairly common set phrase meaning:
"to leave it at that"
So, the whole sentence means:
"I will leave it at that (for you) for today."
For the lack of context, I have no idea what has been going on between the two parties, but luckily as a Japanese-speaker, I do know that one of the two is saying that s/he has done enough (harm ?) for today.
「それくらいにしといてあげる」 can only be said by the "winning/ruling/teaching" party. It cannot be said by the other party. And this is excactly why, in comedy, the phrase is often uttered by the losing side of a fight to get a laugh.
Regarding "doing something in preparation for another thing" as a meaning of 「しておく」, it does not necessarily have to be directly used in the translation. If it is a good translation, that idea will be implied somewhere.
My TL "I will leave it at that (for you) for today." would imply that idea, which could be like "I could do more (harm) to you if I wanted to, but I will not. I will let you go for today.".