Before you buy an office chair, check to see if the backrests, seat, and armrests are adjustable. You should be able to adjust the seat height and tilt of your office chair to your own preferences, and ideally the seat should be comfortable enough to support you comfortably, while letting your body breathe. The most important feature to look for is good lumbar support, as an office chair with a good lumbar support lets your back stay in a position that is good for its spine health.
If you only have room for a twin bed, find one in a chair. A slightly bigger room might be able to accommodate a full-size sleeper sofa, which could function as loveseat when not in use for sleeping. A sofa that seats three people easily can be transformed into a queen-sized bed. Give yourself plenty of time when you are trying out a sofa, so that you get to know it. A mere few seconds of quickly plopping down on one is not going to tell you much about it.