It is kind of expression in Spanish

I think this calls for a free translation. Literally it's saying that Ortega invites us to do all that (all the stuff mentioned in the previous sentence), which is not a little (in other words quite a lot). But to me it would sound more natural to use something like the suggestion above: just on its own, as a sentence on its own (not actually a sentence, but you know what I mean). I think it captures something of the tone of "que no es poco", a somewhat ironic understatement.

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Note added at 26 mins (2013-07-12 22:22:21 GMT)
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You're welcome, Jeff!

I think "todo eso, que no es poco" implies that what the artist is inviting us to do is important and difficult, and "quite a proposition" is designed to reflect that.