Stop completing missions at the point where you're asked to buy a parking lot. From here, you'll be making money solely from fueling and picking the locks on cars. This takes a long time - I estimate it took me about 5-6 hours of gameplay. Make sure you're being as efficient as possible with your lockpicking and fueling, get as many tips as possible, and try not to go insane. If you're looking for other stuff to do while waiting around, I personally ended up also getting the throwing achievements while waiting. You could also try to make a tall stack of trash, or clear all the sand around your lot with Rudy, or paint everything you can. Two things I don't recommend doing are the racing minigame (if you're not consistently very good at it) and flipping cars, since the first costs money and the second might impact how many cars stop at the station.
Once you've made $5000, make sure you've got at least one shelf placed and buy the parking lot, then answer the phone. Complete the next mission, then pay your loan and the achievement will be unlocked.