The pope promotes learning for deacons, but leaves the particular amount to bishops’ conferences:
14. It is to be desired that such deacons be possessed of no small learning about which we have spoken in numbers 8, 9, 10 above, or that they at least be endowed with that knowledge which in the judgment of the Episcopal Conference is necessary for them to carry out their specific functions. Consequently they are to be admitted for a time in a special school where they are to learn all that is necessary for worthily fulfilling the diaconal ministry.
If schooling isn’t possible, a priest may serve as an educator and mentor:
15. Should this be impossible, let the candidate be entrusted for his education to an outstanding priest who will direct him, and instruct him and be able to testify to his prudence and maturity. Care must always and emphatically be taken that only suitable and skilled men may be admitted to the sacred order.
Today would the guidance of an established deacon make more sense?
You do know that once ordained, marriage, even for a widower, is not within Church tradition:
16. Once they have received the order of deacon, even those who have been promoted at a more mature age, can not contract marriage by virtue of the traditional discipline of the Church.
Most deacons I know still work in a career. The deacon’s bishop determines if either an “art of profession” is a problem for carrying out the diaconate office:
17. Let care be taken that the deacons do not exercise an art or a profession which in the judgment of the local Ordinary is unfitting or impedes the fruitful exercise of the sacred office.
Catholic Sensibility 
“Should this be impossible, let the candidate be entrusted for his education to an outstanding priest who will direct him…”
This passage is troubling and continues to reinforce the prevailing priest-centered ecclesiology. Why would a priest be a more desirable mentor for an aspiring deacon than, oh, I don’t know, a deacon? Especially considering how few priests truly understand the ministry of a deacon.
Agreed, Eric.
I wonder if the blinders were also on because of the lack of existing deacons. Most places don’t have that problem these days, I’d say.
I’d like to point out that the best and most fruitful spiritual direction relationship I’ve ever had was with a permanent deacon. The exploration of living a spirituality as a family man, as someone who provides for a family more or less in the world, and all was more insightful and challenging than what I’ve received from the best clergy directors.