Probably just better to ignore JSWolf's comments on the Kindle, he seems to be a pretty intelligent, helpful guy, but he simply can't contain his anti-Kindle bias.....
(Edited to add: of course he's now just posted a more reasonable answer while I've been typing out my reply!
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You can use Calibre to convert between the different formats, but be aware that when you buy books they normally contain DRM and you need to search for Apprentice Alf on the internet for help on how to utilise some plug ins to remove that DRM before you can convert.
It depends where you will be buying ebooks from too, if it's likely that your ebooks will be .epubs, you'd be as well buying an ereader that uses that format. But if you are likely to buy a significant portion of your books from Amazon, the Kindle makes more sense. But you don't need to be restricted to one or other.
As for the links you've posted, I personally wouldn't buy any of them. The age of the older readers (Kobo N647, 1st/2nd generation Kindle's and the Sony 500) and the affect on the battery has already been pointed out to you. I'd personally wait and keep my eye out for a Sony 650, Kindle (Keyboard, non-touch or Touch) or Kobo Touch to see if you can pick one of them up cheaply.
The Kindle Keyboard could also be a good option, though as an example, the one you linked to states "Item needs to be reprogrammed ! > For some reason item can not be registered", which suggests to me that it's a stolen or lost Kindle and that's why it can't be registered? The other one is a demo model and you need to know what you are doing re jailbreaking to have any chance of getting it to work. The Kindle Touch you linked to mentions in the details "Fair Amount Marks Body & Screen", which might be ok but the seller doesn't have any photos of it and is instead using a stock photo, so that's a no, far too risky.
If you can't find any of them cheaply enough, then ok, try the Sony 600 or the Kobo, but buy one knowing that the battery in it is likely to be a bit old and tired. I guess Asawi makes a good point about the old Kobo model, it is slow and feels very outdated, but if it's your first ereader and you can get it for <$30, then it might be fine for you.