It'd really help if you describe your plans for the bike. What will you be doing with it? Where will you be going? From what you've written so far, it sounds like you'll be using it for recreational/fitness riding. The water bottle cages have quickly been recommended. Maybe when you ride longer distances, you'll find that you'll appreciate the various hand positions to switch to with the addition of
bar ends. As for computers, the most popular abilities of cyclocomputers are to show distance traveled (for current trip and/or total amount), time, time elapsed on the current trip, current speed, average speed, and highest recorded speed.