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In fact, if you book some of the recommended hotels, all the other would be in a walking distance for you. If you will find a hotel out from the center it will be super cheap just to order a taxi to go to any milonga (tel. 12 653 55 55). All the locals use taxis in here so do not be worried about the prices.
If you would like to use the public transport that runs also during the night, please check this site. You just need to know your address (not a stop point) to search for a bus or a train.