It seems to me that the average porteño speaks more english than the average non-native speaks spanish, and most will jump at the opportunity to practice their english when given the chance. So, the difficult part I find is having what often ends up being a two way conversation, and getting them to speak spanish so you can work on comprehension etc.
I've found most locals to be pretty forthcoming about correcting your grammatical mistakes (for example when you utter a verb but let it hang in the air, unsure what ending to use) if they know or feel like you're cool with being corrected. I find myself peppering "se dice?"'s here and there in the conversations I've had, which is probably a bit of a crutch but helpful in learning what is right/wrong and working towards a point where everything is more automatic. On their end, if you say "no entiendes" when you don't understand something they'll likely restate it in simpler terms.
As for the dictionary, I can't say that I've seen a single person with one since I've been here, though that doesn't mean they're not used or useful. The time involved in looking up a word seems a bit impractical mid-conversation, though it surely wouldn't hurt keeping one around. Good luck!