Hi C.! Congrats on your due-any-day baby! My kids are 2-1/2 years apart--the baby just had her first birthday, and my little boy is 3-1/2. We had planned on them starting to share a room once the baby was out of the bassinet in our room, but our first few attempts didn't work very well--our older child was already in a big boy bed and didn't really understand that he couldn't get up and put things in the crib to comfort his sister. Also, she wasn't really sleeping through the night at first, and they'd wake each other up. So I couldn't stop worrying and they couldn't stop crying! (I assume your 18-month old is still in a crib, so this might be easier for you.)
We abandoned the plan temporarily, and the baby slept in the pack and play in our room until about a week before her first birthday, and then we moved her into the room with big brother. They've now been sharing a room BEAUTIFULLY for almost a month, and I'm not worried about her safety because she's a 1-year-old and my preschooler is a bit more grown up and doesn't try to put things in the crib. We still move the baby to the pack and play in our room for afternoon naps, so they can each have their own space because my son doesn't always sleep anymore, and I want to make sure the baby does.
We live in a 2-bedroom house, so we never had the option of a temporary room just for the baby, but when she wasn't quite sleeping through the night, I often wished we had one for sleep training. Those days are past, and everything worked out fine after all. I recommend trying out room sharing when you feel ready, but be prepared for it to take a couple of attempts before it feels right. Good luck, and best wishes with your new baby!