I absolutely needed a double stroller in the beginning. After a few outings with the baby in the stroller and the 20 month old holding my hand, I realized for safety sake I needed a double stroller. You need a stroller for all kinds of day to day activities...going to the post office, any store besides the grocery store, going to the mall, or even just to the dollar store. An 18 month old, no matter how well behaved, cannot be trusted to stay by the stroller, and you won't want to have to put the 18 month old in the stroller and the baby in the sling every time you get out of the car...the baby will most likely be sleeping most of the time as you try to get around.
That being said, that is what worked for MY family. My MIL gave us a double stroller, but it was so big and bulky I couldn't fit it in my trunk. I ended up looking for one all over and found one at Burlington Coat Factory for $100 that held up. The car seat pretty much fit in the back seat, it folded small, and my older child rode in the front seat. I would hold on to the handle of the infant seat when we went over bumps to make sure it was secure. I used it for YEARS and even passed it on to my sis in law...it was the best $100 I spent!
If you think you won't mind putting the baby in the sling every time, then try it out and buy one later.