NaVette makes some great points about whether or not to buy a car. I agree that if you are planning on being in Argentina for a relatively short time and don't want to spend a lot of time outside of BA, there's no need for a car. Public transportation is good, if very squishy and hot during commute times. I pretty much leave our car parked in the neighborhood during the week. As I mentioned before, I don't have a garage space, so that saves quite a bit of money. However, if you're going to live in a dense neighborhood like Recoleta, you will need a garage. Our insurance is very reasonable, especially compared to what I'm used to. Buying the car from Car One was a headache of cash exchange and tons of paperwork, but I knew I was getting something decent buying it from them instead of from an unknown private party.
I've rented cars here several times over the years and it has always been a mild to serious nightmare. The first time, I used an local rental company (I don't remember the name, or I would warn you against it), and our car completely broke down three times in the course of a four hour drive to the coast. Since it was late afternoon on Saturday, the few mechanics along the road were closed. Luckily, those pricey "peajes" on the autopista also pay for free roadside assistance, but it still threw a wrench into our plans. The company was totally unresponsive via phone and when we managed to get the car back to BA. Most recently, I rented from Avis in Mendoza, and arrived to pick up my "reserved" car only to find they didn't have it. Again, things worked out eventually, but I had only rented the car for a few days and I wasted quite a bit of time dealing with the rental agency. In addition, the rental fees here are also much greater than in the US, and the fee-included mileage is often limited to around 100 kilometers a day.
If you really are planning on road tripping all over the country, I would buy a car. After the initial pain and stress, it has absolutely been worth it.
Hope this helps! Best of luck in your adventures!