"Por medio de la presente" (which of course means "la presente carta") is typically a formula with which one begins a formal letter, in this case a letter of recommendation (or testimonial or reference). In English I don't think you'd pitch straight in and say "I highly recommend". And "hereby" is not typical of this kind of letter either, in my view. The standard formula (and I speak as someone who has written a fair number of these letters for students) is "I am writing to...".

So I think the natural way to begin is "I am writing to recommend X".

Here are a large number of examples:
https://www.google.es/search?num=100&site=webhp&q="I am writ...

How do you handle "ampliamente"? One possibility is just to leave it out. But it probably ought to be reflected. You can say "I am writing to highly recommend X...". Plenty of people do. But this gives you a split infinitive. I personally don't really mind this, but a lot of people dislike it, and since the object is to make a good impression, it may not be the best policy. So I think I'd put something like "recommend X most warmly" (for whatever it is). But there are many standard formulas you can use, depending on how effusive you want to sound: "in the strongest terms", "without hesitation", and so on.

--------------------------------------------------
Note added at 1 hr (2013-08-29 22:31:06 GMT)
--------------------------------------------------

So I agree very much with Edward about "hereby", but I have the strong impression that "Por medio de la presente" comes at the beginning of the letter. That's my experience, anyway.

--------------------------------------------------
Note added at 1 hr (2013-08-29 22:36:32 GMT)
--------------------------------------------------

Another formula which is commonly used and would get round the problem of how to fit in "ampliamente" is:

I am writing to offer/express my strong recommendation of X...

--------------------------------------------------
Note added at 1 hr (2013-08-29 22:53:57 GMT)
--------------------------------------------------

Browsing examples confirms my strong impression about "hereby": you just don't use it in personal references. It sounds far too impersonal and businesslike. You'd use it in more legal situations, when recommending that someone be granted some concession, for example. It's not actually essential to provide a direct literal translation for "por la presente", and in letters, local rhetorical custom is paramount and trumps literal accuracy.

But "Por medio de la presente, recomiendo..." is perfectly standard:

"Como se elabora una "carta de recomendacion"?
[...]
Por medio de la presente me permito recomendar a C. [...]"
http://mx.answers.yahoo.com/question/index?qid=2007080716463...

--------------------------------------------------
Note added at 12 hrs (2013-08-30 09:49:26 GMT)
--------------------------------------------------

It's one thing to write a statement and another to write a letter. In the former, you probably would start by simply saying "I recommend X". But the latter is directed to someone (even if only "to whom it may concern", and it would seem very abrupt to begin like that. The convention is to start by acknowledging in some way that this is an act of personal communication, and my suggestion is one way (Ronaldo's is another).