It looks to me like your requirements are loose enough that you can choose pretty much whatever you want. I don't know much about the non-mass market ereaders so I cannot comment on those. If you have found you are interested in I would suggest posting in that ereaders specific board area. You will probably get more responses there then here.
Since you are in Brazil and you have mentioned wanting 3g/WiFi I would look at the Kindle 3 and the Sony 950. Both have 3G/WiFi. The Kindle browser can be used with the 3G and WiFi, the Sony 950 browser can be used with the WiFi.
They are both well built and supported internationally. The Sony 950 is only sold in the US but I would think that would be less of an issue since the 350 and 650 are sold across the globe.
They both have a ton of books available for them. The Kindle store is massive and getting around geographic restrictions is not difficult. Use a US address, gift cards, and maybe a VPN blocker and you will be able to buy books in the US store fairly easily. If you are concerned that won't work for you, the Sony 950 is an EPub reader and you will be able to shop at more stores then you can with the Kindle. Note, more stores does not neccessarily mean more available titles.
And frankly, Calibre is very easy to work with so no matter what way you go you will be able to buy books with Mobi or EPub and convert them for your device easily enough if you so choose.
They both have
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