I wouldn't recommend a Late 2012-Early 2013 rMBP. The starting price for those machines was 1700$. They came with SATA SSDs, Ivybridge processors, and actually weigh more than the Late-2013 models. The 2013 revision was one of the largest upgrades, especially considering the 400$ price drop. If you cannot afford the Late-2013 8GB model, you should just get the MBA, because it will be a more powerful computer overall.
The baseline model now comes with 8GB of RAM, so if you get one of those, you should be able to find one at a reasonable price. Depending on the work that you do, a 4GB model should do fine, just don't get a 2012 model. There are much better options out there.
Matt