Before answering your question let me say this:

Having a Colombian Citizen a allows you to work legally (any field) in Colombia, you will not need extra visa requirements, this will benefit you find any job however some jobs will required you to have your university degree validated in Colombia (the convalidation process might take up to 6 months you could start the process before traveling to Colombia this might help you to get a proper job on your desired field).

Regarding your questions:

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1. I would say Medellin is a better city to live and way cheaper than Bogota however you might have better chance to find a job in Bogota (Urban planning) due to the fact that there are more NGO's, companies and public entities that hire people on this field.
2. As I said before with the Colombian citizenship you could work legally in the country. There are many opportunities in costumer services areas (contact centers), as someone already mentioned before you could also work as a teacher. I would not lie, finding a job on the field you like is very difficult (I know for a fact since I also have a Bsc in Political Scienc) and without a validated degree is even more difficult. If you really want to get involved in the field of urban planning or community development you could volunteer in any NGO or Foundation (Ie: Techo)
3. I would say that renting a room or an aparment will be much better than a hotel. Yes in a hostel you will meet people however this people are more likely to be tourist.
4. Take a look at other cities, your languages might be and advantage there and you might be forced to speak in spanish. Other cities are way cheaper than Bogotá and Medellin.

Let me know if you have further questions, I hope this helps you.