Hi Tom, I personally don't know the north coast, but my Wife suggests you look at Santa Marta rather than Cartagena, cleaner, healthier, and cheaper.
Copacabana is only about 20-30 minutes from Medellin, the pain in the backside is that unless we take the back roads, the only option is to use the Toll Road which is 2500COP in each direction, the funding goes to Bello, the next town, even though their residents are not affected by the Toll, this is a bone of contention here and the other towns on our side of the Booth. I live up on the mountain ten minutes beyond Copacabana, so it takes me 30-40 minutes to Medellin, longer during rush hour.
Copacabana the town, needs some TLC, as do many of the towns and villages here, we are unlucky enough to have had one corrupt Mayor after another, they enter the position with nothing and within a couple of years they have a property portfolio and garage of vehicles to rival many rich folk, and in the mean time, nothing is done to improve the town, also their family and friends are given the best jobs in the Town Hall. Why is this allowed to happen? unfortunately when it comes around to elections, they buy votes, with free meals, weekends away, healthcare bills paid, etc, but after they are voted in nothing happens, because there is no money, it is all syphoned off.
Colombian friends here say I should just accept and live with it, but it's not in my nature to do so, so I keep sticking my head above the parapet and having a dig, one of these days I may well receive a visit...
However the ordinary folk in town and here abouts, are really friendly, fincas in the countryside, nearly all which have pools, are very reasonable compared with in the city, because they are mainly on lower stratas, I am on strata 2, but many of the Expats in the Medellin area, insist on living in El Poblado, or Envigado, these are 5 and 6, on my pension, I couldn't live there and have a good lifestyle, here I have no worries, my utility bills and local taxes are so much cheaper.
I can't really recommend any company for rentals or sales, it is so long since I bought my property, people come and go in that business, Fincaraiz is one site that has been around for a long time, and will give you a good guide on prices.
If you are going to look outside the major cities, I would recommend renting, then spend a week just driving around the local villages, to get a feel for where you might like to settle, then ask in a local Bar or Cafe for the local man who knows all, they will put you in touch, and this man will take you out for the day, or however many it takes, and show you available properties many of which are not on websites, and will be cheaper.
Before you go out with him or her, find out what their sales policy is, because their commission may just be paid by the seller, it may be split between the seller and buyer, and if he is a rogue it will be paid by the buyer. Make sure any money is only handed over after everything is signed and sealed at the Notary, there is usually a time limit entered into the documentation for paying, but if you pay direct to the Vendor before it is signed and sealed, there is a chance you wont see your money or your property again.
I believe that you can buy property before you get a cedula, by using only your passport, but you will need a Colombian Bank Account, so your funds are ready to use, many people here don't want to wait until you can transfer funds, so it is better to have them here.
Saying that, it depends on how you intend to transfer your funds, if it is for the sole purpose of buying property, and you may want to sell and take the funds out of the country again, then you will need to transfer the funds using a form 4, this registers the funds to a specific property with the Bank of Colombia, and it is easy to remove those funds out of the country again. However if you have no intention of leaving, then like me you can use a form 5, which allowed me to import the funds from the sale of my property outside Colombia, and use them for general purchases including a property, but you will be taxed when or if you decided to leave, for me it is easier, because my wife as a Colombian could export the funds, as long as my name wasn't involved.
Hope this helps!