Interesting sutta, and a very unusual usage of kāya here. While BB has “of this life”, Thanissaro has “in the body” and Hare has “whole being”. In my translation I originally followed BB’s rendering, but looking closer I think I’ll change it.
The key is in the verses. The operative term is in the final line, attano, “for oneself”. Kāya and attā have somewhat of a semantic overlap, both being used in the sense of “oneself, one’s own personal experience”. It is unusual, though, to find the normal term for this (attā) in the verse, while the more “poetic” idiom (kāya) is in the prose. In translation, I would render them both “for himself”.
Note that in these Anguttara-style verses, the normal situation is that the verses act as a summary or restatement of the prose. This isn’t always the case, but it’s a safe assumption unless there’s a reason to see it otherwise. So where one term is used in the prose and another in the verse, they were usually intended as synonyms.
Midha is not a word, apart from the one highly dubious reference in the dict. The word is sāra-m-idh[a]-eva where -m- is sandhi, and the final -a on idha is as usual elided before eva. In this context idha means “here, in this life”: “only what is essential in this life”.
“Mendicants, a male noble disciple who grows in five ways grows nobly, taking on what is essential and excellent for himself. What five? He grows in faith, ethics, learning, generosity, and wisdom. A male noble disciple who grows in five ways grows nobly, taking on what is essential and excellent for himself.”
“He who grows in faith and ethics,
wisdom, and both generosity and learning—
a good man such as he sees clearly,
and takes on only what is essential for himself in this life.”