Buy an apartment

UPDATE: since the latest 1.0.3 patch, the quick replay has been removed. So sadly, from now on only the first completion gives 9000$, and every repeated run only 3000$.

"Violent Duct" from Gerald. It gives 9000$ and can be done solo in around 3-4 minutes. You have to be level 15 to start this mission. It is currently only the second best mission (as far as I know) to make money, but it's available from pretty early on and you can do it solo.

A few hints:

• do the mission on easy difficulty, you take less damage and still get the same reward
• a gun with high range comes in quite handy, get an AK-47 as soon as you have access to it
• flank them from the left side. As soon as you collect the coke, avoid the incoming enemies and just drive off the same way you came from

This mission was nerfed so it only gives you 3000$ per run after voting for repeat, but you can avoid that by pressing the R2 (quick restart) on PS3 (not sure about xbox, i guess right trigger) after rating the mission. The only problem with that is that you can't buy ammo between your runs. So bring a lot of ammo with you. If you run out of ammo, end the mission, restock and call Gerald until he gives you the same mission again.

The answer is no, you cannot own more than one garage. You also can not own more than one apartment or house. And to get even trickier, you cannot have a combination of a house and a separate garage. I hope this was clear and concise.